Generally, there’s a lot going on in Berlin. Currently, not so much on a group ride level due to Covid restrictions.
Classic meeting spots are Kronprinzessinnenweg (Krone), Tempelhofer Feld, Ostkrone.
Mailing list with announcements for group rides (predominantly German messages, though):
https://www.rennradtraining.de/
Rapha Clubhouse offers many rides (primarily for RCC members, I suppose).
Standert shop rides: Berlin, Germany Club | Standert Bicycles on Strava
Flux RC cycling club:
https://www.fluxmusic.de/flux-kosmos/flux-rc
And many smaller groups/clubs that you might find on Strava or get to know once you’ve been riding in Berlin for a while.
Not sure what can be found on Facebook since I don’t use it.
CX/Gravel: