Forums are productive, useful and fun for me when I limit myself to around 15 minutes a day. If I let myself go all out, it can be easy to spend 2 hours on a forum and find myself wondering where the time went. I've made many real connections which originated from an online forum, so I don't consider it time wasted as long as it is spent well and not just fiddling around.
And even if you do just fiddle around, if the time spent is kept in check, then who cares - it's just good old entertainment. At least it's much more interactive than television.