That doesn't really make sense. A YC-backed exit that nets a "nice signing bonus" doesn't move the dials for YC, and the perception that YC is a farm team for Facebook and Google hiring doesn't help them either.
I don't know if this is what Calacanis is saying (I kind of doubt it), but the risk is that YC is acting against interests inadvertantly. And, again, probably not so much by Facebook stealing!your!ideas! so much as by creating a single locked-up railroad track for bizdev, putting FB at/near the center of a startup's solar system.