It has constantly amazed me that Facebook's mobile app experiences are so, so poor.
At the same time, having watched my friends swear at Facebook on their smartphones and yet continue to use the app, day-in, day-out, maybe Facebook are more clever than I give them credit for; I haven't seen people move elsewhere because of the problems.
I don't use Facebook because it's got a superior UX, I use it because it's the only site where I can videochat with my girlfriend, catch up with my middle school math teacher, and view pictures of my grandparents' anniversary at the same time.
The cost of this demand type is attention. People will keep paying attention because they are invested personally (or they believe other people are invested personally and they want access to the peoplemarket).
You can measure the elasticity by adding anything detracting from user experience (i.e. wasting your user's time when you could automate solutions (e.g. cragislist spam apartment listings you spend 20 hours sorting through manually) or speed up your site (e.g. social website slowness/downtime/high variance in page load times)). You've found some elastic jumps when users start leaving because you're wasting more and more of their time.
People only have so many minutes of focusable attention every day. It's precious and irreplaceable. Don't waste your user's limiting waking attention with errors, manual aggregations, confusions, slowness, or "oops this failed try again and it may work or it may not work and you'll have to try again and it still won't work."
It's not very "creative", but the site is pretty awful on an iPhone. You have to zoom in an order of magnitude to be able to click up/downvote, and the layout doesn't shrink to fit the smaller screen at all.
Facebook doesn't want to optimize the mobile experience. They make far less money from mobile users. Their ideal world would be that people can get their immediate fix on their phone, but for most interactions, they head to their desktop.
They are getting dragged into a better UX. They aren't pushing for it.
At the same time, having watched my friends swear at Facebook on their smartphones and yet continue to use the app, day-in, day-out, maybe Facebook are more clever than I give them credit for; I haven't seen people move elsewhere because of the problems.