Facebook's stock dropped as low as $17.55 in Tuesday trading, its lowest point since the company's May IPO at $38. The stock later finished at $17.73.

Facebook is moving up the expiration date of a post-IPO ban on stock sales by employees by two weeks.

Facebook Inc. also said in a regulatory filing Tuesday that CEO Mark Zuckerberg won't sell stock in the company for at least the next 12 months.

Many of Facebook's rank-and-file employees can start selling stock they own on Oct. 29 instead of the earlier date of Nov. 14.

 Facebook says it's withholding about 101 million shares from its employee stock units to cover taxes they will incur at a rate of about 45 per cent. It will then pay the tax obligations, currently estimated at around $1.9 billion, cash and credit.

Price target cut

Facebook's stock fell to $17.55 in Tuesday trading, its lowest point ever, and closed down 33 cents at $17.73.

Earlier in the day, an analyst for the bank that orchestrated its initial public offering cut his target price to $32 US from $38.

The latter was Facebook's IPO price, the one it hasn't hit since its first day of trading on May 18.

Morgan Stanley analyst Scott Devitt still has an "Overweight" rating on Facebook stock.

Doug Anmuth, analyst at JPMorgan, which was another large underwriter of the offering, also cut his target price, to $30 from $45. And he kept an "Overweight" rating.

Devitt says Facebook is starting to make money from mobile ads.