You can bootstrap as soon as you have one peer. The software is distributed with the addresses of 4 existing servers which are run by the Tribler guys to make this easier but even if they go down people can still send invitations to their friends.

Word of mouth? (This could work well in environs like a university.) Twitter? All one would need is one peer to bootstrap.

It could be using the http://en.wikipedia.org/wiki/Kademlia distributed hash table. I don't know know if it is or isn't, but it's one way to do it.