Shares in Facebook Inc. plunged 11 per cent to below $24 US Friday after the company posted an underwhelming first earnings report as a public company after markets closed on Thursday.

Facebook stock on the Nasdaq was changing hands at $23.85 a share, off by almost $3 at the first chance most investors had to react to the company's earnings release.

The company saw strong growth in its user base, and now has 543 million active mobile users. That's up by almost two-thirds from this time a year ago. Across all platform, Facebook had 955 million active monthly users as of June 30, up 29 per cent from a year ago.

But the company isn't increasing its revenues as quickly, so the increase in its user base without an accompanying boost in revenue is actually squeezing profit margins as costs inch higher.

The company posted an adjusted profit (excluding one-time charges) of 12 cents a share and revenues of $1.18 billion in its first quarter as a public company. A year ago, the company pulled in $895 million.

"Growth is still at the low end of acceptable," independent technology analyst Carmi Levy told CBC News. "We still don't know whether or not this is a one-trick pony, or if Facebook really has a plan to drive consistent growth quarter after quarter."

The company actually reported a net loss of $157 million, or 8 cents per share, in the April-June period, mainly because of compensation expenses it incurred when it paid $1.3 billion in restricted stock and related taxes for employees as part of the initial public offering. The loss compared with earnings of $240 million, or 11 cents per share, in the second quarter a year ago.

Facebook shares are now trading at their lowest level since the disastrous IPO on May 18 at $38 a share. In the days and weeks following that IPO, the shares never rose above that initial $38 price.