People you know who suddenly turn into link sharing, status updating, friend connecting, farmville freaks. Very annoying.
Twitter is a whole different animal. There are no page invitations to ignore, events to opt-out off or people tagging you in photos. Basically Twitter is built on the whole idea of serendipitous finds; you are bombarded with tweets, but only if you choose to, and pick what is good, and ignore the rest.
Facebook makes it a lot harder to ignore stuff. And being able to ignore the stuff that annoys you is the basis of any good relationship.