My newly 8 DS just this week received his first phone call with a friend to just to chat! He finally has a friend that he wants to do things with outside of school. He, like many of your kiddos, showed little to no interest with the other kids in his class as a preschooler up until now. He didn't ignore them, and he was friendly with them and played with them, but he wasn't interested in hanging out with them (except his best friend since infancy).
I think some of it has to do with personality. He is pretty slow to warm up to anyone new, and mom and dad were so much fun anyway.

Some of it likely had to do with the fact that he's a boy; not many boys in DS's schools had great friends, compared with the girls who definitely got into the friend thing earlier. I'm sure some of it had to do with GTness, in that DS was interested in different stuff. His new good friend is also a grade skipper in a school for HG kids, and they have a lot in common, but also they're quite different in many ways. Who knows how friendships work! I wouldn't have predicted this one.
So in our case, it just took time.