A gathering of nerds attempting social contact, likely for the first time. Usually disintegrates into everyone running to the nearest computer to type to one another.
Tommy: Hey Jethro, quit playing WoW its time to meet some folks at the tweetup!
Jethro: Oh yeah I forgotabout that, I can't wait to meet BigBootay421 at the tweetup, but first I need to slay this orc!
To cram a "tweet" on Twitter in Twitter's 140 character limit using an intense level of chat-speak ; a hybrid between the terms "chat-speak" and "tweet."