It is almost back to where the numbers were before that bad server downage, from a U.S. West Coast perspective at least.
Pre outage:
Peak Time (Weekends GMT 20:00-24:00): 150-200
EU Off Time (After work weekdays): 100-120
US Off Time (After work weekdays): 75-100
The following weeks/months after the outage:
Peak Time: 90-130
EU Off Time : 60-90
US Off Time: 40-60
Current:
Peak Time: ~150
EU Off Time: 100-120
US Off Time: 75-100
With the recent Google video and the long run of stability with at least the server coming back quickly when it did crash, some veterans have returned, but I think it is more new players finding the game. My buddy list still seems pretty Red when numbers are around 100 online, so means I need to make more and new friends

My theory is you'll always get that flow of new players since it is a free game, but with higher instability the turnover will increase which will counter act that. I'm sure there is a formula for it somewhere.