Yes. For example, the Facebook numbers are always number of people who've logged in at least once in the last 28 or 30 days (or something) -- I'm sure the G+ people are computing this internally. The fact they don't report it implies it may be a bit low?
Not quite. There's a difference between Google accounts that are "switched on" for Google+ and those that are not. These days also you need to do is sign into Google+ with your Google account to flip the switch, but there is still a distinction.
If you are signed in to google for any reason(gmail, docs, etc.) and are a G+ user you are signed in to G+. They put a G+ specific notification area on the black bar across the top. I would hope active user meant something more than signed up and has checked their gmail account at least once in the past 28 days.
