Facebook. Now I know what a lot of you are thinking, but seriously, I love Facebook. My list of friends is at 190 now, and 99% I know in real life. People I haven't seen or spoken to since we were in grade school. People I thought hated me who have felt guilty for over 20 years for how they treated me - now coming back into my life to apologize and compliment me. People I worked with or spent time with and lost touch, and finding more of them all the time.
BEST social networking site ever. It has brought me untold happiness to be back in touch with these folks, and I have Yellow to thank for it. So thanks, yellowmono!