While it's certainly very crude, if not plain gross, it's not all that uncommon for people to use social media sites like Facebook while in the bathroom.
Many Posekim forbid speaking in the bathroom, at least while one is in the process of relieving oneself.
Is social media use of this type considered speech that is forbidden in the bathroom?
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.
From the Ben Ish Chai Shana Alef Parshas vayetzei 10 talking should not be done but to make noises is fine ,it is saying words(letters) that is problematic.It seems that texting words should be fine.
