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Is it cross platform (eg, can GOG users play with steam users)?
Is there a centralized matchmaking server (eg, you get a list of running multiplayer games) or is it only possible to directly connect by entering the host's IP?
Finally, what is the overall multiplayer population like? is it possible to find games at low to mid levels?
This question / problem has been solved by Sarisioimage
It seems to be cross-platform.

You don't enter IP, instead you get a list of servers hosted by other people (and can make your own of course).

There are always some games running.