[alex_c]

I fall squarely into the second camp, and most of my friends know it - they don't expect a reply from me on facebook, and for something time-sensitive they contact me through email or phone.

Everyone has different means of communication they prefer. Some people like to stick to email or IM, others vastly prefer phone, some people always let their phone go to voicemail and call you back at their leisure (or not at all). There's always going to be some give-and-take, Facebook is just another piece in that puzzle.