I'm 25, so not a teenager, but: a) I'm feeling a bit younger than my age would tell, b) I was a teenager only a while ago.

Still I was not in any way the most representative example of my generation. Around 13 I started becoming a loner, more and more. I felt weird with people, especially socializing and spend most of my free time alone playing PC, watching movies and reading. Natural extension of that was that I have never got a Facebook account (to be honest I created it once, as my favorite Polish band was postponing the release of their record and Facebook was the only place where they shared some info about it), no Twitter, nor any other. Sometimes I got this surprised looks and questions "You don't have a Facebook?! Why??". I just replied that I don't feel a need to, which was true. I also didn't have any single photos of myself and I felt all those sites were mostly about sharing photos. As I wasn't socializing too much in the physical world, I didn't feel the social pressure too big - or maybe I just didn't give a fuck, as I always did about most people advising me how to live my life - it's always hard to tell the difference to me. When I grew up (which came a little late in my case) and saw how uninspiring and stagnant my life teenage has been, I decided to go out and meet people. I re-thought participating in some social media then, but turned it down. I think main reason was I saw people all around being almost addicted to it and I thought it's not worth my time. It seems it stayed this way to this day, as I still don't feel any need to create account on any of those popular sites and feel it'd be waste of time. That is one of the reasons I ended up here. I just looked for some more rational, less photo and rituals oriented place.

To be honest I've never heard of the few sites you mentioned: YikYak, Medium, Whatsapp. It only shows I still haven't caught up with my generation : ).