You have friends. A script can check your friends, and make suggestions based on that.

You won't have the full graph that Facebook does, but does that really matter? I suspect the best predictors of who your friends are would be people already in your network.

Isn't friend recommendation mostly stuff like "people who follow you/you follow also follow X"? Maybe going one or two more follow hops, but not so far out that you need a global view of the social graph. If users of a decentralized network publish their following and follower lists, it should be pretty easy for a local crawler to check those hops.