Losing one or two of the big ones (fb or twitter) would be a glaring hole in their offering and it also seems like the same thing would trigger companies to cut them off; namely, too much success.

I know they're smart guys (by proxy of YC) so they must have some answer to what happens should one of the handful of social networks that matter cut them off, but being on the outside looking in, it sure does look like a precarious position.