When AOL first entered the market, you could only send E-Mail to other AOL users. Eventually, faced with the potential of users leaving their platform for more interoperable (if less full-featured) alternatives, AOL relented and made their email system a proper, standards-based utility.
Now, granted that scenario is not a great example as email existed as a standard long before AOL came into existence, but I do think that as the number of users on Facebook/Twitter reach their inevitable peak, those companies, too, will be faced with the need to open the gates on their walled garden.
It's interesting that, thus far, both companies have delayed such a situation from arising by buying-out the other platforms people have been moving too (Instagram, Vine, etc.) and incorporating them ever-more-tightly with the parent platform. At some point, though, I think (hope?) this strategy will prove untenable and open interoperability will once again rule the web.
Call it crazy speculation, but I could foresee a resurgence of RSS/Atom eventually doing to Facebook/Twitter what SMTP/POP/IMAP did to AOL. If you think about it, the only thing that RSS/Atom was ever missing from becoming what Facebook/Twitter are today is a central aggregation point in the cloud. A smart startup that pulls a user's Facebook/Twitter feeds and mixes in any other RSS/Atom, as well as allowing users to post back to Facebook/Twitter/Atom could potentially spell the death of proprietary timelines.