Friday before the event @robin.berjon.com organized an event, but I was not able to join. Maybe he can give us indsights about the place they went to.
Saturday, there were more than 8,000 attendees, so at the end of the day, they spread across the city for social gatherings. My friends and I did not sign up for the Tailscale-organized one, where I heard most of the atproto people went. Instead, we went to a random pub, and someone at the entrance asked us if we came from the FOSDEM venue. Then, they gave us bracelets, and GitButler sponsored our beers inside.
I modified the settings of matrix.atproto.fr to federate with the fosdem.org server. Now, anybody can join a FOSDEM room through the atproto Matrix server using atproto OIDC. It is not useful right now, but it can become useful later, as each physical room has a dedicated Matrix room.