I found out through a web search, because I knew my friend's legal last name. I was lucky to figure it out. People disappear from online communities every day without a trace. If you want your online friends to know in case something happens, consider a social will.
Mine is pretty simple. I wrote down a list of forums, blogs, and IRC channels I frequent, and also contact info for some of my friends there, and gave it to my son. There are also websites set up with instructions or even notification systems. Here are a few instructional links: