‘We went to the 1D concert and it was so fun!’
This seems to be new way of using ‘It was fun’ or ‘It was so much fun’. Then again, if you were Penny (Patricia Hodge), Miranda’s mother in the sitcom Miranda, you would say ‘Such fun’!
The word ‘fun’ is traditionally considered a noun, as in ‘We had fun’ or ‘That was fun’, and the use of ‘fun’ as an adjective has, for some reason, long been regarded as improper: ‘We had a fun day’ … ‘It was so fun’.
According to the dictionary, ‘fun’ as an adjective is correct but informal. Therefore, ‘so fun’ can be correct, but ‘so MUCH fun’ where ‘fun’ is a noun is a better choice. If you use ‘fun’ as an adjective, you should say ‘more fun’ or ‘most fun’.
It seems to be fashionable amongst teens (a friend of mine says her daughters use it all the time), so if you don’t want to be thought of as someone trying to be young and trendy, best stick with ‘So much fun’!