There are two directions this could go. The first is let users post html so they can litterally create anything. I'll talk more about security for than in a bit but in general there are different approaches. The second is to have set kinds of elements like myspace. The options there would be a fancy graphical system or an xml like language that would translate to html and javascript you control.
So there are many ways to appoach security with custom html. One is to restrict script tags. A way you can have script tags is to put the user pages on a seperate domain preventing calls to the main site's backend with user credentials in the cookies. But I would definately be down for some html only pages. Using a subdomain in the case of html only would just be extra prevention.
No social site right now seems to let you make a page that you have real control over besides maybe github.io if we call github a social site.
A more complete mindmap of options that could do this:
Cutomizable profiles
HTML
HTML Only
Allow javascript
cytu.be like user approval for js
just runs
[jade/pug like editor]
Controled elements
WYSIWYG editor
Custom markup
Saved posts
Markdown
with javascript
with user approval
without javascript
[main domain / subdomain]
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