“We believe that some of our users, particularly our younger users, are aware of and actively engaging with other products and services similar to, or as a substitute for, Facebook. For example, we believe that some of our users have reduced their engagement with Facebook in favor of increased engagement with other products and services such as Instagram. In the event that our users increasingly engage with other products and services, we may experience a decline in user engagement and our business could be harmed.” That is from a Facebook Annual Report. Alyson Shontell writes in Business Insider that surveys have confirmed the above.

So why isn't Facebook "cool" anymore? A social media expert opined to The Verge that it has to do with Facebook just no longer being a space that serves kids, as people now tend to look for more intimate places to share items with a handful of people, like Snapchat. There's a sense of privacy there, and it meets a need Facebook has grown too big to serve. This doesn't mean teens are deleting their Facebook profiles, of course—they are just using it less. But Zuckerberg needs not worry too much about it. Teens are still rabid Instagram users. So the Facebook man can still monetise them on mobile devices.

For more:
http://www.234globe.com/story/130/Teens+Are+Bored+With+Facebook.html