I’ve made some friendships here and we’re friends on FB too (the Jellies that I’m friendly with who are also on FB). Fluther is a place where I can be totally honest about some topics that may be really personal, may be embarrassing to discuss with people I know or may be about the people I know. An example is discussing work relationships or relationships with friends and family.
Fluther is also a place where we sometimes discuss mundane stuff like what we eat or our personal habits. FB is not for that, usually. If I have a dilemma, Fluther is a place I can get objective responses from.
FB is where I maintain friendships with people, share photos, hear about friends’ milestones, events, silly quotes, and am in groups of various types (crafts, cats, groups for people who live where I live or have kids in the local school system). Just like @NerdyKeith, I never accept friend requests from people unless they’re either people I actually know, friends of friends or a rare few who I know from groups.