Brent Simmons makes some great points that he can put much more eloquently than I can.
Long story short, social media sites come and go. Even social media itself could end up being a passing fad. So could your email provider. While you may post everything on Facebook and use Gmail in 2014, there's no guarantee that either of those things will still be here a year from now.
You need your own domain, your own blog, your own email. You need to own your online identity. When you build and control everything yourself it can last as long as you want it to.