Quite a good post and I may even make it into a wiki page unless you want to !
Things that make online users go up!
- A user with cookies disabled browsing the site. Each request will create a new online user.
- A search engine, same as above, depending on the engine
- People grabbing content with the likes of wget, each get is a new online user
and many more where cookies are either rejected or not taken.
the best way is to tail your apache log file. This will reveal IP addresses and User Agent strings, you'll easily be able to tell if it a googlebot or alexiabot or whatever.