If you are going to delegate your freedom of association to someone - which is obstensibly what you do by using someone else's instance rather than setting up your own - make sure it is someone you trust not to keep you from speaking from people you wish to speak with.
The larger and more inclusive an instance you want to build, the more minimal the choices become about blocking and banning.  This is why small niche communities are probably better.
@maiyannah In theory, these networks could scale up. I think it's important for communities to grow organically over time, though. The Web is comprised of niche spaces, many of which emerged over time. No long-lasting community simply emerges overnight.
@deadsuperhero Small relatively speaking to the meganodes or silos.  I mean a niche community is always going to be smaller than Twitter or Facebook.
@maiyannah Otherwise it would just fragment into a hierarchy of subcommunities within a walled garden.