It lost 84 cents, or 4 per cent, to close Thursday at $20.04.
Another 5 per cent decline would bring it to $19 — half of its initial offering price of $38. The stock traded as low as $19.82 during Thursday's session.
The stock has been falling since Facebook released quarterly earnings last week for the first time as a public company.
Investors were disappointed despite second-quarter results meeting Wall Street expectations, with revenue one-third higher than last year.
Facebook began trading publicly in mid-May.
But since then, investors have been concerned about its ability to keep increasing revenue and make money from its growing mobile audience.
Many analysts hold positive long-term views.