Do grad schools google their students before acceptance?
I have no doubt that some grad schools google applicants.  I'm pretty sure that defamatory things others say about you will have close to no effect on your admission.  It's the things you say yourself that might give you trouble.  If you're openly pro-nazi on your facebook page, that'll be a problem.  

And, unfortunately, if you take a bold public stand on one side of some issue (say the Palestinian state) and someone one the committee feels strongly the other way, it could still be a problem.  But don't worry about others' comments.  Everyone knows about trolls.