Hands down without a doubt, Facebook. From my earliest up bringing I was taught by family and teachers alike that social media (namely Facebook and Myspace) were pretty much the source of all evil and drama and they should be hated with a fiery passion and avoided wherever possible. The effects of that upbringing are quite clear today because I hold many biases against Facebook, some of which I am sure I could justify but I know if forced to explain my standing I could not vindicate my entire views of the site without bringing up this up bringing as justification at least for not giving it a fair chance. Still knowing this I simply cannot bring myself to change my views of the site, where many other social media outlets have numerous and some even countless practical applications Facebook in my eyes is nothing more than a massive drama factory that the world may well be better without. Or at least would be better off if they did things different anyway.
At any rate I will have to be using it to some limited extent in Marketing for my Indie company but I should like to think I won't enjoy, still who knows, I may drop some of my own impractical biases after using it for awhile... Or I could pick up new ones...!