When is this field updated? What is considered activity?

It's mainly used for user online tracking. It's updated when the user first logs in and when the ajax activity tracker pings the server every 2 minutes.

Thanks - I am trying to write a SQL report to show the number of logged on users per day (I do not use the core SiteLog) - this may help.

WRT to the Online Activity - if we are only using the ActiveSocial online indicators - do we still have to enable Online Users in the core and the Scheduled Task or do you handle it differently?

Yes and No. Active Forums uses some aspects of the Core Users Online, but this is changing soon. Active Social doesn't require it at all. In my opinion, Active Social tracks it much more efficiently. The disadvantage is that we don't track anonymous users.

Ok, thanks.

Is there a way to get the date and time the user last logged on? I'd like to have a view that shows users online and includes "online since ______"