I make a last effort, and if they still don't reply, I delete that person from my contact list and find someone else to talk to - someone reliable.
For all you rejected aspie's - delete these unfriendly people from your contact list, and if they do reply eventually you could even delete their message without reading it. It's mean, but if you need closure and no longer want to continue a "friendship" that makes you feel bad about yourself when that person does not talk to you, than go ahead. Don't feel guilty. Being a little evil is good sometimes.
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Some of the threads I started are really long - yeay!