Originally either via Lifehacker or Google, then a slow, slow crawl from lurking on the Green to lurking on the Blue to finally ponying up my 5 bucks.
Questions in the Human Relations category
I was in high school - introverted, bookish, and overweight, but finally ready to start coming out of my shell. And I had no idea how to do it. So, like any geek of my generation, I sought my answers on the internet. How do I meet people, how do I talk to people, how do I keep a conversation going? And lo and behold, I found this bright green website with all these questions from people like me, and everyone else was giving them answers, answers which I noted dutifully.
I think I must have read every single question in the 'human relations' category. And I started making friends, and I started going to parties without being frozen in the corner, and I brainstormed topics of conversation prior to meeting with people. Little by little I started to have some semblance of a social life. Maybe it would've happened anyway without Metafilter, but reading those questions gave me tips and confidence to come out into the real world.
I read questions on AskMe for years afterward, and I learned so much (not even just about social skills, but all sorts of topics). I didn't even notice the Blue for a long time. In total, I think I probably lurked for 5 or 6 years before getting my account - I got my account because I discovered that one of my real-life friends had an account! Someone I might never have talked to, were it not for Human Relations answers on the Green.