From Bespoke Investment Group:
...Below is a chart of Facebook's short interest as a percentage of float (SIPF) since it IPOd back in May. As shown, short interest peaked at 19.4% of float just before the stock made its 2012 low, and SIPF has declined significantly since then. The most recent short interest figures for Facebook showed just 5.8% of its float sold short. Early on during the stock's current rally, short covering provided a big boost. Unfortunately for Facebook bulls, there are now a lot less shorts out there to potentially cover, which makes it more difficult for the stock to rally. Gains going forward will have to come from new buyers stepping in instead of short sellers throwing in the towel.
While in 2008 we were still pitching shorts on GM and Fannie Mae in the $2.00 range, after >90% declines, we could do so in a public forum because they were each worth less than zero. Facebook was never worth less-than-zero.