There's over 200,000 that have signed the honor code.
One metric that forums and social media sites keep is unique visitors in the past month or past year, that's probably the best way to determine how many are active.
My activity comes and goes, some months I'm pretty active, then I can go for 6 months and not make a single entry. So I don't think we want to write people off as inactive too soon. Still, there's definitely a large number in that 900,000 that are likely never coming back.