I just stopped having a filter. When I think of someone, I just fire off a message. The message doesn't have any warnings on it like "oh I know it's been a while" or "you might not remember me". I write to everyone as if we are best buddies who just had lunch last week. People I've known since the age of 4, to people I've known for four days.
If I see someone I know at a wedding, I just go and talk to them about whatever we have in common. Normally someone we know.
I really think it's the guarded, tentative, "you don't have to talk to me" that turns people off. Of course people are free to not talk to me, but I don't lead with that. If you lead with that, people feel awkward, like "is he just being polite?". If you just pretend you are best buddies, people play along and they end up quite comfortable quite quickly.