What if someone post something about you on Facebook can your employer use it against you?

I believe they can not use such postings about you directely in the UK.
(Nothing to say they can't use it to start an investigation thou)
Just a reminder to say they can use something you have said on line, if it is commercially confidential, compromises company or even national security if your buisness is in defence. If you say something that brings disrepute to your companies public image. Be careful what you say in the public domain as you can be subject to company discipline but also sued for damages.