I agree that restrictions are often liberating, and likely played a role in the success of the big social networks. But I disagree about email not being able to have the same kind of restrictions. It absolutely can. All you need is an email client that controls what you can put in the email body when you're composing an email. Most email clients allow pretty free-form messages - from plain text to hundreds of lines of HTML. But they don't _have_ to.
By the same token, there's nothing in HTTP or any other internet protocol that restricts the content of a Facebook post. Those limits are imposed by Facebook's clients (and presumably APIs).
Email has a widely understood delivery protocol and identities. That's really all you need to let people publish and subscribe to each other. The rest is just UI and marketing. (Which, of course, is no small feat!)
On any of the platform, there's only so many ways you can format a post. Facebook for example lets you do plain text, dress it up a bit, or attach some piece of content with optional text (a photo, video, or site embed), and then presents your post in a standard way after that.
It seems restricting when I put it all down on paper, but that restriction is downright liberating when you want to get something out quick. Choice can be overwhelming, and for better or worse, the lack of freedom in Facebook's post editor makes it much easier for folks around the globe to pick up and use. Email won't ever have that, and that's okay! Different tools for different jobs.