I don't think a blockchain Facebook will kill steemit. That would assume that all steemit users would flock to facebook.

This is a complex world where you can use Steemit without using Steemit.com. You can use a blockchain fb and steemit.com at the same time. You can even use yours.org and steemit simultaneously.

The reason steemit is here and growing daily is that people appreciate the platform and community. As those two continue to improve steemit will continue to flourish.