Are they really my friends?

Maybe try to contact them through social networks like facebook? I also haven't really seen many of my friends either but sometimes I would just message them asking how they are and what they've been up to this summer, and most of the time we just fall back easily into normal conversation like other friends, what they're doing, family, things to do and so on.

I guess my advice would to just make small talk first, maybe go out for coffee to catch up over a meal?

I would but in the past I've started the conversation so it should be them that should contact me but they have not.