I'm using betonline.ag. They accepts bitcoin. There are hundreds to thousands of online users most of the time.
Those freerolls are killer, though. You can spend hours in one and still not be ITM. I haven't tried their cash games yet as I haven't won any freerolls to go test them yet, :p.
You can deposit min $20 in bitcoin and play on gtd tournaments there are 10x less players than freerolls.