iPhones weren't ubiquitous for at least a few years after the initial release. Parents of teens and younger weren't the first adopters but it came on relatively quickly. The Social Network movie came out in 2010 which Facebook had already opened up to general public in 2006. Also was 2006 that News Feed was released and introduced the concept of doom scrolling and constantly checking into things. Respectfully, if you did not experience it's popularity until ~2015 I do not think you are representative of the overall trends; they hit a billion users in 2012 and that certainly wasn't just college students.