Thinking it through what might take down FB is a portal that links in your other social services, if all said services are willing to create APIs to work with each other.
So, for example, you'd post your pics to Instagram/Snapchat, your ramblings on Twitter and your longer blog posts on Steemit.
The FB replacement then would be where people manage their friend lists and other things I mentioned, while the feed itself is populated from Instagram/Snapchat/Twitter/Steemit.