Killing Floor Servers

you should be able to find at every time at least a couple of servers with players

steamcharts.com

I saw on SteamCharts and still have a couple of people playing :)

Yeah, it's still pretty active now.

Yea there is a lot of servers. Sadly the best Workshop servers died (KFManiacs Servers)

Be sure to play only in whitelisted servers to level up. And if you play solo, don't use blacklisted mods. And there is a map (Don't know if still works) to level up really quickly. Because it has been a coouuuupleeee of years since the game. And with KF2 it will... yeah.... gotta go fast.

There are more players on Origin