Three bases for friendship.
1. Utility: the most common basis of friendship. It exists between two people who are mutually useful to each other. This is the friendship of Facebook, LinkedIn and the recruiter.
2. Shared pleasures.
3. Goodness: The need for reciprocity is abandoned. You provide affection and support without payback. This is for friends who have learned to trust eachother over time.
From Heather Brooke, Poking Aristotle