Traditionally when people have thought of Facebook and their defensiblity, they've pointed to its ubiquity and the size of its network – at last check they had something like 1.1 billion active users. People reasoned that Facebook would continue to dominate social because it's the one place that has profiles for all of your friends. All other social networks would be forced to plug-in to the Facebook ecosystem. But as Facebook’s defensive purchase of WhatsApp shows, this is no longer the case. Users are bouncing from social network to social network. Social apps are much, much less sticky than initially thought.

Benedict Evans and others have pointed to the seemingly minor but incredibly impactful fact that any newly launched social app can easily tap into your mobile phone's address book and instantly build out a network equal to -- or better than -- Facebook's.

This wasn't a big a issue when most users accessed Facebook through the desktop site, but now that most users access it through their mobile app, Facebook's unbundling has accelerated.

More and more users are migrating to WhatsApp for messaging, Vimeo for video, Instagram for photos, Foursquare for location sharing, etc. And there are niche players internationally that are focused on badges, stickers and other features valued in those communities. There are now dozens and dozens of social apps in the app store with more than one-million downloads.

Facebook's strategy of running the social ecosystem seems to be shifting more rapidly than they had planned. Because of the mobile phone's address book, the approach of plugging social apps into Facebook may be losing steam. Instead of just letting them plug-in, the better approach, it seems, might be to buy them.