Facebook's stock fell to $19 for the first time on Friday, losing half its market value since the company's initial public offering in May.

The stock dipped 87 cents, or 4 percent, to briefly hit $19, just minutes before it closed the trading day at $19.05. Facebook's shares ended the week down nearly 13 percent.

Facebook hit the $19 milestone a day after the expiration of a lock-up period that had previously prevented some early investors and insiders from selling their shares. Stakeholders who owned a combined 271 million Facebook shares before Thursday can now sell their holdings.

A breakdown of just how many major Facebook Inc. shareholders sold their stock this week won't be available until next week at the earliest, when sellers must disclose such transactions.

Facebook's stock has struggled since the company's mid-May IPO. It closed its first day of trading barely above its initial offering price of $38. It has been below that level since.

The stock has been down on 38 trading days, up on 25 days and unchanged on one since its initial public offering.

Investors have been concerned about the social network's ability to increase revenue and make money from its growing mobile audience. Many analysts, however, hold positive opinions of the company's long-term prospects.