I totally agree with you. So many times I had that kind of conversation; while in their thread they seemed amazingly witty and funny to be around... messaging them inworld they're completely cold and disinterested.
I reached a point where I honestly don't even bother anymore... I rather hang with my small group of friends even though I'd actually would love have a bigger group of friends. It makes me sad because on the forum I've met my best friend.
A way to completely throw me off is: when you meet for the first time someone and they get immediately clingy as hell, like if them logging on second life or their happiness depend on my friendship. That's a HARD PASS for me. LOL