if I'm interested to send a private message to a certain user from serverfault, how can I do that?
This has been discussed ad-nauseum on Meta Stack Overflow (where Stack Exchange itself is discussed) and has been declined (several times, if I recall correctly).
See this question on MSO for excruciating detail.
Server Fault does currently however have a chat room going on http://chat.meta.stackoverflow.com . If a person wants this sort of thing that is probably the place to go hang out (Closest you are going to get).