@Greyling I discovered the same things you did when NPCs hang out on my Sim's recommendation. They don't tend to follow up on their initial connection, unless you do as @Stormkeep suggested and bring them together for social events.
@Stormkeep, your idea about using the Club feature to help NPCs develop closer relationships is a great one. I'm going to give that a try, because my current "fave save" has several groups of NPCs whom I like to see hanging out. I've used cheats to add friendship points to these groups, but I'd rather have it happen more organically.