Sometimes. It's nice to see where some of my good friends from back in the day are now. But with a few, as denise.w. mentioned, there is no longer anything in common, so I deleted them. A lot does change over time and sometimes it's best to leave some folks in the past.
As far as any exes, no, I never add them. I find it kind of weird. Especially, when they reason they are exes is something terrible. Who'd want to be friends with ex-lover that hurt you? Not me.