Well, anyone who looks at Facebook or reads their email at work is technically misusing company resources. But when smokers take a 15 minute break every hour, I think a company who fired you for poking someone would find itself staring down the wrong end of a tribunal.
I think that comes down to expectations. If an employer decides that Facebook is a distraction, communicates with staff that the site will be blocked, AND an employee hacks around the block in order to access the site - that could be a dismissal offence (more likely a warning, but if it's part of repeated behaviour...). Compare that to the (albeit, sadly, probably more likely) scenario that you are probably referring to - a boss deciding he wants to sack someone and using Facebook use, which the business has never actually defined one way or the other, as a reason - and yes, that's a silly business move.
