You probably already know, but just in case (and also for any possible new players reading this): if you use /p <playername> you can also see to which server someone is connected.
Didnt know about /f online. thats helpful. Although I think two commands just makes it harder again to remember all. Dont see a reason to not show something like: /f 1: Hasorko (online SMP1) 2: PenguinDJ (last seen SMP3) something like this. The /p I knew, but thats already to much work to go through all the names, just to check what friends are on .