>> You can build your own version of fb/twt etc.
You couldn't build your own version of fb that can view posts from your friends.
You can setup your own email service to view emails from your friends who are using @gmail.
That's an important distinction.
The distinction is that 99.9% of consumers and an increasing number of organizations don't want to do it in house and would rather let someone else do it.
Again, it's not about capability, it's about what people actually are doing.