Well we have a "ministry of cartels" (Kartellamt) and other laws that are supposed to prevent monopolies from forming. This never worked though.
Monopolies and Companies pretty much having the power of a state including their own laws, culture, etc. is one of the reasons I have been a socialist for a long time. However I think regulating companies with tight-nit laws, only for the big ones to maneuver through it without actually abiding by the law and small firms suffering because of all the legal hurdles.
Like I said, I think the solution has to be either the free market where facebook just gets replaced by a better competitor (like hive xD) or there needs be a state-sponsored and controlled social media, where freedom of speech is upheld and every citizen has a right to participate. The state controlled social media being in a fair competition with multiple other social media platforms would be my favorite outcome.
Still I wish you good luck with your endeavor even if I would chose a different path.