One downside to Mastadon for me so far: the servers I join keep shutting down. 
I figure joining the main place will increase my odds of longevity! Also got #activitypub going on my #wordpress website, but still haven't configured after installing last year
I guess I'm just usually adverse to all social no matter who runs the platforms 
But I'd also love to see the fruition of a truly open web, so 'be the change you wish to see'. 