I believe it was my former brother-in-law - an early adopter blogger - who kept bringing up things he read on MetaFilter, so I checked it out and started to read it regularly. Then I used MeFi as a way to keep up with 9/11 in that epic thread, and stuck around afterwards too. Not every day, but enough to catch the new signups fairly quickly after they opened back up.
I work on the 17th floor of an office building in Arlington, VA. On 9/11 I could see the Pentagon on fire from my office window. The entire office evacuated to our apartment nearby, but we didn't have cable. We could only watch awful quality fuzzy TV, and none of my normal news websites were accessible. So I spent most of the day on the blue, refreshing the thread and learning about the rumors and facts as they happened, and reading people's reactions. It was a sort of lifeline for me that day, and I'll always remember it as being a huge help in sorting out what my feelings were, and as a way to distance myself a bit from what was going on, so I could make good decisions on behalf of the group (my first serious crisis as the person in charge, as I was the most senior staff member present).
Secondary benefit: MeFightClub - which has made a hugely positive mark in my life, and has become a lifeline of its own over the last few years.
Thanks, truly, for a great place to hang out online.