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In this May 16, 2012 photo, the Facebook logo is displayed on an iPad in Philadelphia. Facebook's stock has fallen below $30 for the first time. That's down 20 percent since its stock began trading publicly on May 18, following one of the most anticipated stock offerings in history. (AP Photo/Matt Rourke)

Today: Facing tumbling stock prices, Facebook and Zynga go on the offensive, but only the Menlo Park social network finds relief on Wall Street. Also: Stocks bounce higher, Verizon changes its pricing plans.

Facebook and Zynga try to change spiraling fortunes

Facebook and Zynga, tied together through a partnership that led both Silicon Valley companies to bountiful initial public offerings, have suffered together on Wall Street since Facebook stock began trading. Tuesday, both companies made moves in an attempt to ameliorate doubts in their business plans, but only Facebook managed to push its stock higher.

The Menlo Park social network began a public-relations push as the "quiet period" surrounding its IPO expired, finally allowing the company to counter doubts about the effectiveness of paid advertising on its website. General Motors' decision to pull paid ads from the site and downgraded revenue projections had caused doubts, so the company produced two studies it said proved the world's most popular social network can drive commerce.

A report partly commissioned by the company through digital metrics company ComScore showed that Facebook campaigns drive traffic to retailers, with Starbucks showing a 38 percent increase in offline purchases by Facebook users who saw an unpaid post on the site and an unnamed retailer experiencing a 56 percent rise in online purchases from users who were presented with a post that was sponsored. An internal Facebook study showed that more than two-thirds of advertisers received at least $3 for every $1 they spent on Facebook ads, with nearly half making $5 for every $1 spent.

"There's great value (in Facebook advertising), and we've proven it through countless case studies," Facebook corporate communications manager Elisabeth Diana said Tuesday. Brad Smallwood, head of measurement and insight at Facebook, was even more blunt in an interview with the Wall Street Journal, saying "it's a myth that Facebook advertising doesn't work."

Facebook's stock has suffered on doubts about its ability to increase revenues since its much-hyped, record-breaking IPO last month. Facebook scored a record valuation of $104 billion by selling shares at $38 apiece, but the stock has fallen as low as $25.52 in intraday trades since.

Facebook stock moved higher after Tuesday's PR blitz, however, increasing as much as 2.9 percent before closing at $27.40, a gain of 1.5 percent.

Zynga, meanwhile, attempted to sate critics by expanding its pricey mobile acquisition, "Draw Something," to new markets, including to China, where it will be the San Francisco company's first mobile game. The move also includes a partnership with one of China's most popular social networks, Sina, which is a microblogging service similar to Twitter.

Zynga paid $180 million for "Draw Something" creator OMGPOP as the game, which gives users a topic to draw on their mobile device and has other users attempt to guess the topic, broke download records. With U.S. downloads slowing, Zynga will begin offering the game in 12 new languages with pop culture references specific to the targeted countries and market the move with help from Latin American pop stars Jennifer Lopez and Enrique Iglesias.

"With native pop culture references and our partnership with global superstars such as Enrique Iglesias and Jennifer Lopez, the game will be even more fun for more players," David Ko, Zynga's chief mobile officer, said in a news release.

Zynga's stock was slaughtered on Wall Street despite the news Tuesday, falling lower than $5 a share for the first time. Cowen analyst Doug Creutz said in a research report Tuesday morning that Zynga users have abandoned the company, with daily active users dropping 8.2 percent last month, sending the stock in a tailspin that resulted in Nasdaq suspending short sales through Wednesday.

"We believe that interest in Facebook-based gaming may have reached a negative inflection point as more casual gamers migrate to mobile platforms," Creutz wrote.

While the company waits to see if the expansion of its popular mobile game helps keep the bears at bay, the stock fell 10.3 percent to $4.98 at Tuesday's close. It has now declined 39.8 percent since Facebook's IPO and more than 50 percent since its own.

Stocks move up more than 1.1 percent, led by solar

While Zynga was spiraling Tuesday, Wall Street was moving higher, resuming the forward push it experienced last week after a brief downturn Monday.

All three major U.S. indexes rose more than 1.1 percent Tuesday thanks to a late push after Chicago Fed chief Charles Evans said he would support efforts meant to push job growth, and Spanish bond yields lowered.

One of the most profitable sectors Tuesday was solar, which got good news for the U.S. industry leader, First Solar. The Arizona company landed a large project in Australia and decided to keep its German manufacturing plant open, sparking a rally that sent its stock up more than 21 percent and also helped competitors such as San Jose's SunPower (SPWRA), which increased 8.8 percent.

Apple (AAPL) rose less than the overall market one day after announcing its latest updates, increasing 0.9 percent despite analysts applauding its latest moves and reiterating price targets much higher than the company's closing price of $576.16. One of Apple's new partners did exceed the overall market's gains, as San Francisco-based Yelp rose 4.2 percent Tuesday. Google (GOOG) continued its Wall Street descent Tuesday, falling 0.6 percent as it continues to suffer from being in the cross hairs of Apple with the Cupertino company's Maps offering.

Verizon to switch to shared-data wireless plans

The biggest consumer news from Tuesday was a significant change in the billing policies of the nation's largest wireless carrier, Verizon. The company announced it would change its plans to offer unlimited phone calls and texting while having limited data plans cover up to 10 devices.

The plans will allow users to spread their wireless data over more than one devices, such as smartphones and tablets, and will start at $90 a month. Costs will likely rise for customers who do not currently have unlimited calling plans, and customers who are locked into unlimited data plans from earlier contracts will be forced to relinquish them when buying a new phone, unless they pay the full, unsubsidized price.

Analysts said other wireless carriers are likely to follow suit, and the industry pushed higher Tuesday on Wall Street. Verizon increased 0.9 percent, AT&T rose 1.1 percent, and Sprint went up 3.2 percent.

Silicon Valley tech stocks

Up: SunPower, Yelp, Advanced Micro Devices, Intuit (INTU), Adobe (ADBE), NetApp, Nvidia, Cisco (CSCO), Tesla, Symantec, Facebook, Slunk, Juniper, Hewlett-Packard

Down: Zynga, Jive, Google, Gilead, Electronic Arts (ERTS), eBay

The tech-heavy Nasdaq composite index: Up 33.34, or 1.19 percent, to 2,843.07

The blue chip Dow Jones industrial average: Up 162.57, or 1.31 percent, to 12,573.80

And the widely watched Standard & Poor's 500 index: Up 15.25, or 1.17 percent, to 1,324.18

Check in weekday afternoons for the 60-Second Business Break, a summary of news from Mercury News staff writers, The Associated Press, Bloomberg News and other wire services. Contact Jeremy C. Owens at 408-920-5876; follow him at Twitter.com/mercbizbreak.