Of course they have incentive. They need to get users while they are young (i.e. before they are using a competitor). Teenagers take and post a lot of photos. If you get them to store them on Facebook you lock them in for life. Then you get value from them in 5-10 years. If Facebook only thinks short-term like you suggest they will die. And I think it's been proven Facebook has always thought long-term (e.g. prioritising UX and getting users over advertising for a long time after launch).