I know I've been a little behind on the forums lately, but I didn't think I was THAT behind that I wouldn't recognize these people. WAY before my time!
When I notice people who haven't posted in a long while, or those who register, but never post, I always wonder what's happening in their lives, how they're doing, etc. To the first group, I say "don't be a stranger. Stop by and say 'hi' once in a while." To the second group, I say "Don't be shy, come on in, make yourself at home. You won't find a friendlier, more welcoming group than this one."