This is an insignia that apparently appeared inside Mark Zuckerberg’s “hoodie” during his D8 interview (SF Weekly reconstructed it). As they say in the article this symbol appears nowhere on the site or in public Facebook materials otherwise:
Here’s the clip where Kara Swisher of AllThingsD jokes about it:
It was struck by it; it seems odd. What you think? Is it playful or sinister?
Update: An identical hoodie put up for sale on eBay by a Facebook staffer is now going for $510 (as of 11:26 Pacific time on Sunday):