Facebook shares dipped to a new low Thursday as the company unlocked 271 million shares for trading. The stock dipped below the $20 mark for the first time Thursday. By mid-day, shares were hovering just below that mark — a drop of nearly 6 percent from the day’s open and nearly 50 percent from its initial offering price of $38.

The company has had a rough time since going public and has never recovered from its glitch-plagued opening day.

As the Associated Press reported, Facebook has been down on 36 trading days, up on 25 and unchanged on one since going public. The company met expectations in its first public earnings report, but questions persisted about whether it will be able to mon­etize its mobile platform and continue growing its business.

The company faces the challenge of being able to mon­etize its free service without annoying its users — something that’s particularly difficult to do on a smartphone or tablet-sized screen.

Facebook recently began testing placing ads on the mobile devices and computers of users who haven’t “liked” the products being advertised, Bloomberg reported. Those ads will be labeled as “sponsored,” according to the report.

Despite a rocky start, many analysts are still optimistic about the company in the long run, saying that its plans for mobile advertisements are maturing but have shown early promise.

Early investors in Facebook looking to sell Thursday are likely trying to make some quick money, and they may soon be joined by others who hold restricted stock. Employees who’ve received Facebook shares as compensation will become eligible to sell their shares by the end of the year — the Wall Street Journal reports that the number of shares still poised to hit the market exceeds 2 billion.

(Post Co. Chairman and Chief Executive Donald E. Graham is a member of Facebook’s board of directors.)

Related stories:

Facebook stock could see shift Thursday

A Facebook court battle: Is ‘liking’ something protected free speech?

FTC finalizes Facebook settlement