There's a bug in the friends list that shows up when you have more than 255 friends (go figure from a social site.) Cull your list to current, active friends and you will be able to message them.
A fast way to do this is to go to your profile, expand your friends list, right click on the one you want to delete and select the top option-open link in separate tab. Delete the friend and close the window. This is much faster than going to their profile and back to yours.