OK, try to be calm.
Have you received a letter to say you are facing a disciplinary? When is it? What exactly did the letter say?
What is the Internet usage policy at your work? Do they have one? You need a copy of it in writing, ASAP. Do other people access the internet whilst in work time? Can you prove this?
You also need a copy of their policy in relation to IT security. What does it say about sharing log-ons? Have the company breached their own IT policy by failing to provide everyone with their own log on, thereby making you vulnerable?
While you are at it, you might as well ask for a copy of the staff handbook, disciplinary policy and grievance policy.
What do they mean by a Facebook activity log? Have they hacked your account and printed it off? Have they looked at your account as a 'friend' would be able to do because of your privacy settings? What are your privacy settings? If they have managed to hack your account and look at information which isn't publicly available to everyone, that is a gross invasion of privacy (and a breach of the Telecommunications Act) which I would urge you to submit a grievance about, (and threaten legal action).
In terms of accessing Facebook on the day of your disciplinary investigation meeting - that is clearly bonkers - why would you do that? When I log on to Facebook, my account remains logged on until I actually log off. So even if it is a week later, if I type in Facebook into the address bar - it will automatically bring up my account. I think this is the most likely explanation here. Is there any way that you can show on your phone bills that you accessed FB via your phone, not your work PC?
I would be asking for a copy of the IT expert's report in advance of the hearing, in order to allow you to consider it. I would be sating that failure to do so is a clear breach of the rules of natural justice. I would also ask for your own IT expert to be allowed to examine the computer in advance of the hearing. Naturally you will expect your employer to be reimbursing you for the fee in the event that their findings corroborate your account. Do you have anyone who could do this? It may worth shelling out for someone to do it. In any event, I bet they will refuse, but you need to ask the question.
You need to get some legal advice, pronto. Your house insurance may cover it.
I would be putting the requests for written copies of the IT policy, internet policy, disciplinary policy, and grievance policy and request for your own examination of PC in writing to them, recorded delivery, ASAP.
When it comes to the hearing, make sure you bring a friend, ideally someone who has past experience of this type of thing. I would ask (at the time, not in advance) that you are allowed to record the proceedings. If they say no, I would be making sure that your friend's smartphone has accidentally knocked itself on to voice recorder mode.
Sorry to hear you are going through this.