I live in Uruguay, right across the border, same median income basically, and a lot of people have smartphones: there's a chinese smartphone with Android made by ZTE for Movistar that is given away for free, and Samsung Galaxy Mini is also free with 15 dollar contracts. Huawei also has a very cheap smartphone.
I have a Samsung Galaxy Ace as well, which is the most popular smartphone as it is among the cheapest handed out by the state telephone company (Antel).
Another very popular smartphone is the very old Galaxy 3 with Android 2.1 or 2.2 (NOT SIII - http://en.wikipedia.org/wiki/Samsung_Galaxy_3 )
I can second the fact that Facebook is king here, I have 3 teenager cousins and 2 nephews about dysoco's age, and they use Facebook a lot. Twitter only to follow Justin Bieber and One Direction as well :) .
I also see a lot of ask.fm links, no idea why.