Private though it isn't. All your 'friends' can see what you put. How many of those friends are what you would call friends in real life?. One of them has reported you.
Unfortunately, this country has become race obsessed. People of the uk have been groomed by the media and society into believing that any name calling even non race is some kind of hanging offence. The days of calling someone four eyes now result you in being banged up.
The fact is, away from the internet and in one to one situations or at family/friend parties no one gives a damn. But in real life people are scared to say anything. It is very interesting that it is only mainly in this country where people are now being arrested for saying what they think or making jokes. I would love someone to make an exact joke about say the disabled or fat people that has been on Little Britain, get arrested for it and then show the courts up by having the clip played in court.
I'm no expert on in the ins and outs but my first thoughts are a section 4 intentional harassment/distress/alarm. I would guess a fine if it is your first offence. It wasn't aimed at a particular person?
Shame on whoever reported you. No life.