Reddit's value comes from the users, and especially from having users willing to moderate themselves. Twitter's value is that it's where the people are, especially public figures. etc.
In general the interests of the users and the interests of the corporations running these services are not aligned. Corporations are incentivized to spend as little as possible to accomplish their product goals, especially ones that struggle to make a profit like these guys.
What other services do you think suffer from being too centralized or might be in danger if we don't see a pivot? Reddit is basically a mega-forum in place of a million independent forums, Twitter is a smaller blog in place of tens of thousands of dedicated web presences/blogs, etc.
The first one that comes to mind for me is fandom.com (aka Wikia), especially as people close their self-run wikis to be hosted there. Such a shame to see something as important as communally-built knowledge being sucked up by a corporation.
