Interesting, but I doubt it cuz social site emails don't allow emails to be grabbed from their site via external email readers, etc. BR I bet still has their employees keep their corporate emails and internal email accounts.
And companies want emails to be extracted, so they can filtered via contacts, spam, viruses, etc. Also people at their desks can organize emails into folders, search them, print them, etc. Let's not forget many programs are setup to extract data from emails and create reports, email others (eg: customer support emails). Furthermore, let's not forget email alerts on mobile devices or sending out mass emails via outlook or creating internal meetings which office people do all day.
Also, it's easier to tell someone to email me at "blahblah@happybunny.com" then "add me on fb, joe schmoe, then email me".
When facebook allows people to read emails through external readers and download them - perhaps it might make a diff.
n