Due to the low level of support for HTML5 in older browsers should one (or could one) use any part of HTML5 at this point in time for a new website that will have varying users with the full spectrum of browsers (and versions) to access the site?

EDIT: To fill in some more info here. The website in question, for simplistic comparison, would resemble fb. I am not talking a social website just one that will reach that type of vast audience (anyone can join) and will be an interactive portal (not just a site full static content being displayed).