Due to the niche audience most FOSS games have, it can be a bit difficult to find enough players to run a decent multiplayer session.
Therefore there are some efforts to get like-minded players together at certain times or for special events.
A good place for asking is the #libregaming channel on IRC. There are usually some people interested in playing Hedgewars and on Sunday evenings there is a good chance for a larger round of Unvanquished these days. Maybe we can also get a group of Veloren players together soon.
Another recent effort is the onFOSS LAN event, which tries to get people together on some fixed dates and organizes tournaments / runs servers specifically for these events. It has taken place twice now, and was quite nice both times.
But these are just two examples, please post other good places or times to get together and play some FOSS games :)