I know this is fuzzy, but I imagine Facebook notifications being sent via e-mail and if Facebook is "installed" either on the device or via opening the browser, opening the e-mail would deliver you to Facebook, or just display as a regular e-mail otherwise. Taking this one step further: if I deliver you a to-do list item, it would open in my default to-do list app making e-mail more of a platform or launcher than anything else.
If you ever figure out how that would work, let me know. I've tried and haven't been able to connect all of the dots connecting those sending the e-mail and those receiving the e-mail as a platform, how it interoperates and where to start. I'd use the hell out of this though.