Facebook. I know, I know, it's a plague upon humanity, but I've been able to keep track of some old co-workers that I would have had to work a LOT harder at, if it was even possible to do so without the FB. But really, what I love are....
...Those people who your positive relationship with them never changes. Like, you get along thick as thieves, don't speak for 2 years, and then it's just la la la like nothing happened.