[ExpendableGuy]

"Social" isn't a fad, but channels dedicated to being "social" might be. This is pure conjecture, but I think a major component of Twitter's success was that participating did not require you to use an official Twitter client. Ubiquity spurred growth. It's an example of "social media," not a "channel for social."

Another major component of Facebook's decline was their lack of a mobile strategy. Again, pure conjecture, but as mobile use in teens increases, the mobile experience matters more -- the Facebook app was unusable for a long time, and that probably had an effect on growth and reputation.

In my opinion, the biggest risk for Facebook is the paradigm shift in how we share photos. A big component of their growth in 2009 - 2011 was their dominance in the photo sharing space. They're the number one photo sharing site in the world. Now photo sharing and picture taking are a mobile experience (e.g., Snapchat, Instragram), being the number one photo sharing site isn't as big of a deal. The purchase of Instagram was smart.