You know how some shops used to reward their 10,000th customer with champagne and party poppers and a new bathroom suite (fir eg)¿
Well, you could do similar with the millionth (2millionith) member?
I was going to suggest reward the billionth messge posted, but the odds are probably skewed in my favour. 
So, bunting and confetti and biscuits and gin, then?
Just think of the flurry of join-ups.