I play on pokerstars and I see people from all around the world but mainly from two large groups:
- eastern europe(mainly russia)
- latin america(mainly brazil)
If I take a guess I would say around 60% of players I see are from these two groups.
Right now i am facing a ton of brazilians and russians, a few ukrainians usually gets caught in the middle of this.
But all this varies depends on the time of day.
The reason Russia and many east Europeans show up is because in Russia and some other east European nations, you can't legally play in-person poker but it's legal to play online.