Legal definitions of libel are not relevant here. Most corporate disciplinary policies have clauses about bringing the company into disrepute. Whether what you say is "true" or not, making statements which could damage your employer's reputation could be deemed gross misconduct and therefore bring the possibility of dismissal.
Complaining to your mates in private about your manager is one thing. Publishing it on the Internet where it can be read by millions is quite another.
As someone who spends part of my working life in HR in aviation, I can confirm that Tight Slot is quite correct - most HR departments don't have the resources to monitor the Internet. But if posts are brought to their attention, it is usually by other staff who do not like to see their company slagged off in public.
When you send an email or post on the Internet, your are taking legal responsibility for publishing a document. You must therefore exercise care and common sense. In my experience it is not management that will "prosecute" you, it is your fellow workers.
RHRP