Mrs rhod I had a similar experience when I joined LSE in 2010 as a mature (commuting) postgraduate. I can't tell you how shocked I was as the contrast between the glossy brochures and some of the LSE Facebook groups eg the "will that f**king girl in the front row put down her hand" group.
I so wished I hadn't looked - it was a sort of grim sleazy underbelly that I wasn't supposed to see! I felt quite intimidated.
But when I arrived, I found lots of normal people, and it was ok. I think it's best to stay away from Facebook until you arrive if possible, and then meet actual people once you get there. I hate the casual cruelty of some of these online "groups".
I rather meanly comforted myself with the thought that all the students who were posting horrid stuff were screwing up their online footprint, which would come back to haunt them when they started looking for graduate employment!