These unwanted forced design changes will be one of the main drivers of everyone off-platforming to their own social network.
Snapchat has gone through a number of major, controversial redesigns. It's definitely not going anywhere (at least among its target demographic). Facebook is far more entrenched than Snapchat; its target audience is less quick to switch platforms as well.
Look at Digg: two-fold cause of 1) UI changes, and 2) user perceptions of centrally controlled, biased, and influenced information flow.
Digg was thought to have unbreakable critical mass.
Now, Reddit and Facebook are both going down the same road, with user-hostile UI and opaque information sourcing.
People aren't dumb, and they've already noticed and made up their mind about it. Now they're just looking for somewhere to jump.
I recently interviewed two kids no older than 20 for a user study. Not techies. One of them doesn't use Snapchat for privacy reasons. Other one thought about it and said, "I don't know why I use it, because I know that it's not really private." I didn't even prompt them with any questions about privacy or control, I was just doing a cursory "what apps or social networks do you use?"
More and more people are saying, "I know it's crap, but I'm on there because everyone else is using it." It means they're already looking for another place to go.
Nowhere near it. Digg peaked at about 40 million MAU in 2010. That's two magnitudes less than Facebook.
Facebook adds a Digg-equivalent to its MAU every month.