Support "social" sharing

[hsoi]

Whereas the "Activity" category is intended to track things that come out of UIActivityViewController (which can be but isn't always "sharing"), have a way to support specific sharing tracking.

For example, I worked on a project where the "product" the app created was shared via a totally home-brewed sharing mechanism, not UIActivityViewController. As well, in some of my own apps I've used iTellAFriend, which is "social sharing" but through other app mechanisms.

Then there's things like Google Analytics supporting the tracking of social stuff: https://developers.google.com/analytics/devguides/collection/ios/v3/social

So... it's all about doing that sort of thing