I am bit cross

There is a very good reason for the rules in grammar, including the apostrophy and the standard of spelling is often appalling, but I would never point it out to someone on FB, although I might make an answering comment, using the incorrect word properly spelt. I have no problem with people not knowing the plural for words such as gymnasium as not many of us were taught Latin at school.

Yes, correcting another person for a grammar mistake or a spelling mistake is very rude.It happens in lots of places, including GN.Some have no social awareness.

I hate bad grammar, spelling etc but it has to be understood that when anything is typed on social sites it is generally speed or 'fat finger syndrome' which we all suffer from and is therefore acceptable as far as I'm concerned. It is far more important on public signs, leaflets, magazines etc

I am going to buy one of the mugs...brilliant.

Yes you are right - typos and autocorrect to add to our own misdemeanours