I'm pretty firm in my belief that one of the great marketing moves of the 21st century is convincing people that email is not a social network. It's almost on par with making people believe that diamonds are romantic and a necessary part of the marriage ritual. I think this has been helped by the general lack of innovation in the email space. From pretty basic mail, we ended up with a few (very surprisingly few) emai…
There's nothing that prevents a non-Microsoft calendaring system from supplanting it, other than on agreeing on the protocol. Email has MIME, there are calendar information formats (ical, IIRC), and various MIME handlers. The primary block was that Microsoft owned the corporate desktop, and competitors, until Google came along, couldn't agree on interoperable standards. Facebook and Google win by being Web based and…
Google and Apple both have integrated calendar and email systems. They just use separate apps instead of having everything in one silo.