One problem with the social graph (or Web of Trust, as it was called by PGP/GPG advocates) is that The Bad Guys can stand up billions of fake identities that all vouch for each other (q.v. "link farm"). If you bias simply for link quantity, The Bad Guys win.
Yes, the graph will be isolated. So bribe (or threaten) a few real people in desperate straits to vouch for some fake identities (or pwn their Windows machines to do it). Poof! The fake island now has "legit" links.