It's a gamble on the idea that people rely so much on the existing content and connections on social networks, in conjunction with the addictive nature of social media, that any attempts to move to a different platform would fail because anything new will always fail expectations of what's considered normal content-wise these days. People leaving sites like twitter complain about the lower quality and advertisers get scared off, but all the owners of these sites pay attention to is the people that come crawling back because nothing else is capable of meeting their expectations right this instant.