The younger Teens Vs Facebook Traffic

I never knew Facebook is loosing the traffic from the teens. So i did a little Google search and found a similar article here in this link 
http://istrategylabs.com/2014/01/3-million-teens-leave-facebook-in-3-years-the-2014-facebook-demographic-report/

Comparing the difference of Teen Facebook users 2011 Vs 2014. The US population for teen are increasing but the Facebook users are going down.
So are they going towards more on Vine, instagram and other more visual sharing apps and social media sites ?

I found this another infogram in this Forbes link :
http://www.forbes.com/sites/parmyolson/2013/11/12/heres-where-teens-are-going-instead-of-facebook/

wechat,vine, snapchat are growing in a faster pace than the Facebook and other traditional social media sites (Facebook and twitter became traditional already in a decade - A very phenomenal growth in the internet).

So My question is How much should Facebook concentrate on developing countries vs developed countries ? In developing countries if Facebook lead with similar visual sharing apps, they could stay ahead in the game and I think they might not loose the traffic like they did in US.
Why is the younger teen's preferences changes so fast ?? or can you say a little about the brand loyalty among the younger teens ? and how to develop brand loyalty by an existing traditional company ?