The two that I used to maintain personal connections and communicate in ways worth archiving were Myspace and Facebook. For Myspace I was a teen. Same with Facebook. My connections were from school so if either disappeared I'd just have to talk to them the next day. Also both had attained critical mass at some sort of scale by the time they popped up on my radar, and were fairly new ideas so it's not like I was able to shop around. I just went where my friends were.