I think it is partially based on trait compatibility, as well as social need vs. other needs and probably also how good of a relationship the sims already have. My Sims have friends that will stand around and do nothing but chat for half a day and others that scurry about doing other things within just a few interactions.
What else is on the lot to interact with and the traits and/or mood the sims have that make them like doing certain things probably factor in as well. My Sims have one friend that if we get together to go work out (the gym) always shoots straight for the punching bags which are in a different area than the other workout equipment where most of the sims go. Sims seem to have preferences... lol
