Facebook (FB: 18.25, -0.84, -4.39%) shares plummeted to a new all-time low on Friday as concerns continued to mount about its ad platform and as lock-up expirations over the next year threaten to weigh on the social network's value and stock.
Shares of Mark Zuckerberg’s company fell more than 4% Friday morning to a lifetime low of $18.23. The stock has fallen about 52% from its initial public offering on May 18.  
BMO Capital Market cut Facebook’s price target to $15 from $25 on an “underperform” rating Friday morning, citing uncertainty around Facebook’s highly-criticized ad platform as well as deteriorating sentiment about the Silicon Valley tech giant on Wall Street.
“Street FB sentiment is now much worse than advertiser sentiment,” BMO analyst Daniel Salmon said in a report. “Consensus sequential revenue growth of +4% may be a challenge in the seasonally slow 3Q, and if results are below this level, another leg down in valuation is likely.”

Many fear that the expiration of restrictions that had prevented early backers of the social network from selling their shares could threaten the value of Facebook's stock. 
BMO lowered its third-quarter sequential revenue growth to flat, reflecting quarterly sales of $1.19 billion from its earlier view of $1.2 billion. The consensus is calling for sales of $1.24 billion. It also lowered slightly its view on Facebook’s non-GAAP earnings in the current quarter to 10 cents from 11 cents, a penny below the Street.

In addition to lock-up woes, BMO said that many direct response marketers have been questioning the value of Facebook ads. However it added that many small to large advertisers also seem optimistic that Facebook is working to address the issue. 

“We heard several comments like ‘it’s not yet clear that it performs like Google search,’ or other lead generation/bottom-of-the-funnel ad models,” Salmon said.
While paid media spending and return on investment remain uncertain, BMO did note that there seems to be “a lot of enthusiasm for mobile sponsored stories.” However, user acceptance of the new ad platform is still unproven.

BMO also noted that Facebook’s display market should “spur incremental spending shifts,” building on real-time bidding inflection points. For Facebook, Salmon said that may mean new demand that could materialize in the fourth quarter of 2012.
Facebook has been trying to cozy up to advertisers. On top of the sponsored stories, the company announced earlier this month that it would launch promotional stories, or ads that will show up in the News Feeds of people who have yet to “like” a brands page.