Up until this just happened, I had never thought about this either. My family knows I post, but they don't know what websites to go to.
I have now exchanged my personal info with a few of my online friends. They will notice if I'm not here and I'm sure they will call if they are concerned. I'm positive now that if something would happen, they would notify everyone else.
It is a good idea to let someone know what to do. Never in my life did I think I would actually find real friends on the internet, but I was wrong. When you post almost daily (as alot of us do) and you share personal stories, sorrows, and exciting events...you come to feel like you really know these people and you do care about them.
Some of you have said that no one will notice if you stop posting. I'm sure that's not true. There are ALOT of posters who I recognize, remember things they post and will wonder where they have been, if they disappear. Just because I may not have PM'd with them or personally posted with them, does not mean I wouldn't miss their posts. I'm sure that is the case with alot of people here.