Ubuntu SE - I seriously believe a lot of our people are there, and I blame management for trying to be democratic instead of putting their foot down.
ServerFault - I see questions on there that should be here. I wonder if we can talk someone into advertising us there. I see quite a few questions on there that are small home network, permissions, things that aren't serious server admin questions, that would be, imo, better fielded here
New Tech - most unix people are hardcore oldschool, meaning mailing lists and irc, also forums.
Unknown - just in general I don't think all the unix geeks know about it. Spread the word.
This site does have relatively little daily visits, but actually 95% of questions asked are answered, so that's not really a problem. (Compare with, say, ask.debian.net, which is an official Q&A site. It sports a whopping 21 people who ever gave any answer...)
The best way to boost a site is to use it. Answer, ask if there's nothing to answer and spread the word.