Here's a shiur talking exactly about your question (haven't listened to it) I would define texting and using social media sites to be basically the same thing.
http://www.yutorah.org/lectures/lecture.cfm/782704/Rabbi_Aryeh_Lebowitz/Ten_Minute_Halacha_-_Talking_Texting_and_Eating_in_the_Bathroom (in English).
Some sources discussing something similar to your issue:
Is writing considered a hefsek during Tefilla? If it is than it would seem that writing would not be allowed in the bathroom either, as it is considered like talking.
See this answer (Siman 46) by R Chaim Zonnenfeild who rules that one can not write in a place where it is forbidden to talk, the footnotes on this page and the next discuss it more at length and bring sources, feel free to look them up
http://hebrewbooks.org/pdfpager.aspx?req=46247&st=&pgnum=25&hilite= (in Hebrew).
R' Volf Greenglass, who was a Mashpia (spiritual mentor) in the Lubavitch Yeshiva of Montreal, rules that one is allowed to write in some cases when one is forbidden to speak.
http://www.chabad.info/images/notimage/27570_en_1.pdf (Page 43/44 [of the margins of the document]; in Hebrew).
However a question that comes out of this: Is texting considered writing, which I don't have an answer to.