Oh no, what a shame it's half term. Hope she'll be able to relax and not dread going back.
I don't know if this will be much concilation or not (probably not), but this is extremely, extremely common amongst Year 7 girls. Whilst it has obviously developed into bullying- and at that age school is their world, thus the world has ended- friendship fights amongst that age group are so, so common. It's actually quite difficult to deal with them, because by the time it has been passed to the behavioural team (hmmmm, maybe this is just my comprehensive) and they have managed to contact the girls, the story has often changed anyway. Also, because it can sometimes be just, 'she gave me a look and I knew what she meant' you are often not dealing with much that is concrete, but that may mean a huge amount to the child.
I don't mean to play this down, as I've never seen anybody as devastated by anything (divorce, a husband's affair, death of a close relative) as a Year 7 or 8 girl in the middle of a friendship fight- anybody who says 'school days are the best days of your life' is a complete idiot. However, it may help you to know that this happens to most girls during the course of those two rocky years and it's worth considering what your daughter actually wants. Is she more upset by the bullying, or because she wants to be friends with the girls again?
In terms of the facebook stuff, I have to say I agree with everybody else. Keep the information, print it off as evidence and the school should deal with it properly. Though consider carefully what your daughter wants, as the consequence of what sounds like a very similar incident in our school this year was exclusion for the two Year 7 girls involved.
If you want more information, please PM me as I don't want to share it on an open forum.