We have entered a new phase, and that's the "friends" phase. A is now obsessed with playing with his friends and when I offer to go to the park or do other fun stuff, his first question is "with whom?" ( well, he doesn't have a perfect grasp of grammar yet, but that's what he means) and if I say "just us" he loses interest -- I don't take it personally though, there's still plenty he only wants to do with me. If I tell him we're going to see one of his friends that day, he gets so excited, I can use that to get him to do pretty much anything, including napping!
How early did your kids really develop friendships? When did you meet your best friends?