I have made friends with some people on Twitter by following them after someone else RT'd them and just making casual conversation. Doing this in non-creepy ways may take several weeks or months, but it is possible. Another way I meet people there is by joining in conversations that include two or more people I follow.

I think it is much harder on Twitter with locked accounts.

I have no idea how to do it on Facebook and I only add people on Facebook who I've met in meatspace.