I have yet to find a social network which is actually accessible. The Google thing (circles?) was never actually useable, it was the biggest horror show of all. m.facebook.com was basically the only website that was ever really accessible. All the other players, including the "free and morally superior" alternatives couldn't give two fucks about people with disabilities, which reflects nicely on the fact that they are actually not an alternative, they are a playground for misguided developers...

Fact is, if you are launching a social network which is not accessible from the get go, you are part of the problem. You have no moral high ground, you're just playing around and widening the digital divide, leaving people behind.