The stock prices of once-hot social media companies likeFacebookcontinued falling on Monday, shaking the faith of investors and employees as the growth potential of social media did not turn out as expected.

Facebook shares on Monday fell to 18.80 U.S. dollars in opening deals, less than half its initial public offering (IPO) price of 38 dollars in May.

Speaking to employees earlier this month, Facebook Chief Executive Officer (CEO) Mark Zuckerberg acknowledged the anxiety over the company's stock performance.

According to a report by The Wall Street Journal, Zuckerberg said the stock's performance was "painful" to watch but investors will be paid off in the long term as the company introduces new plans.

But doubts over the young CEO also intensified as investors blamed him for missteps, such as pricing Facebook's public offering too high.

Analysts and critics have been criticizing Facebook for not devising a well-grounded plan to generate revenue from its mobile business as an increasingly number of its users logged in the social networking website on smartphones and tablets.

Zuckerberg is also taken as immature and disrespectful to investors as he stuck onto his style of wearing sneakers and hoodie in the company's first investor meeting last month.

Investors and analysts have started to talk about whether Zuckerberg should step down as CEO and hand over the company to a more experienced executive, the same way as Eric Schmidt served asGoogleCEO instead of co-founders Larry Page and Sergey Brin in the Internet search giant's early days.

According to Bloomberg, Facebook is the second-largest post- lock-up-period decline among companies that have gone public since January 2011 and its most important partner, social gaming giant Zynga, got the title of worst performer.

Lock-up period refers to a predetermined amount of time following a company's IPO during which early investors and employees who are given shares are not allowed to sell their holdings to keep from flooding the market with shares.

On Monday, Zynga's market cap stood at 2.28 billion dollars. It is less than the company's assets which, including cash, receivables and real estate, totaled 2.7 billion dollars at the end of the second quarter this year.

Zynga went public last December at 10 dollars a share. Its shares reached almost 16 dollars in March but plummeted to around three dollars after the company announced disappointing quarterly results in late July.

The company is also facing at least two major lawsuits. Shareholders have filed insider trading lawsuits against its management team, alleging executives made false and misleading statements in order to liquidate their personal holdings at high prices four months after the public offering.

In another suit, video games giant Electronic Arts (EA) alleges Zynga copied EA's game "The Sims Social."

Analysts have turned negative about the company, saying customers are getting bored with Zynga's once-popular game and questioning whether it could deliver another hit.

Meanwhile, Zynga is also struggling to keep its engineering teams on board. According to documents filed with the U.S. Securities and Exchange Commission which were published last Friday, Zynga is setting aside stock options, which has a current value of about 122 million dollars, for employees in an attempt to keep them around.

On Aug. 10, John Schappert resigned as Zynga's chief operating officer, after 15 months on the job. Meanwhile, some companies in the Silicon Valley started seeing a "flood of resumes" recently from Zynga engineers looking for jobs or trying to raise capital, technology news site AllThingsD cited sources in a report.

Also on Monday, some early investors of daily-deals company Groupon are reported to have sold or significantly pared back their holdings in recent months. According to a report from The Wall Street Journal, at least four major Groupon investors, including Silicon Valley veteran investor Marc Andreessen, are heading for the exits.

Groupon's current market capitalization has dropped to around 3 billion dollars, shedding more than three-quarters of its value since its IPO in November, 2011.