One can argue that replacing real-life relationships with digital ones is unhealthy.
And digital lives are more easily monitored by governments -- not just the American or European governments, which is already bad enough. This too is dangerous.
Being able to go into a community of like-minded people translates in the meat-world to meat communities where your neighbors might have a completely different perception of reality than each other. This too is problematic.
I understand now that there is no company behind it. It's open-source and developed with good intentions, but as the saying goes, the graveyard is full of good intentions. I deeply distrust anything that entices me to reveal personal data in a medium monitored by the secret police.
How much these objections weigh for you depends on your priorities. Non-corporate, open-sourced social-media isn't significantly different for me to assuage my very real Internet privacy terrors.