I'm talking about the main site: stackexchange.com, not any of the specific sites. How is this site doing? Are people using it?
So far so good -- traffic is steadily up. But you should use stackexchange.com, because it'll let you...
Search all Stack Exchange sites (and the trilogy)
See leaderboards per site and track your position on them
Browse questions across multiple sites in a tag set
Have new questions in given tag sets emailed to you
And of course it is a searchable site directory with stats and place to see the most interesting questions across the entire network.
It's good stuff.