The other day I got a facebook notification on my phone, which said something along the lines of "You have 4 new messages". Of course, thinking it was from my friends I opened the app to look at them. 3 of my 4 "messages" were notifications for friend requests from people I had never met. The last one was a photo someone had posted of cake she'd baked (not to me specifically, just in her feed). To someone sitting at her desk at facebook, looking at an engagement metrics chart, the notification would seem to have served its purpose - another data point, another person enticed to open the app in response, engagement maximized. But of course, this was deception. I found this experience distasteful enough to disable notifications entirely - probably another data point for their metrics team - and annoyed enough to complain about in an HN comment.