Because what people like to do with their phones, en masse, is open up the Facebook app and thumb through their news feeds.
From fusion.net extending on their discussion of 'dark social' sharing.
Some may notice I've started to re-post to Facebook. It came down to an inevitable fact: the amount of visibility for spreading material is hugely different if shared, or not shared, to Facebook.
If you're in marketing or media, it's hard to ignore and I guess the take-away is to learn how to do it well. I personally have mixed opinions on Facebook still. Google may have always been my gateway to the Internet, dynamically filtering my searches into an order only they knew. However, Facebook is filtering content my friend's write and link to, so I'd much rather decide myself who, and what, I want to see. But realistically, Facebook is just an advertising business so they have to re-adjust to show what brings in the money. Twitter would appear to be the solution but it's also starting to dynamically shape the timeline so I'm not optimistic. The idea of a paid-communication platform sounds ideal (no ads, I manage myself) but I'm under no illusions that most people wouldn't pay for the privilege and are quite happy to give up their privacy in return for the free News Feed. And I guess I'll take the approach 'if you can't beat them......'
For me, well I'm writing for myself so this just means there's more chance of someone just reading it which suits me just fine :)
Note: while I do post to Facebook, I don't actually go on Facebook so please comment below :)