This isn't a TAAT but it is inspired by a recent thread on this board PLUS many others on MN over the years
Some parents of onlies have got to stop thinking of our children as special snowflakes If they're good at conversing with others it's not because they've had plenty of interaction with adults (mumsnet myth) - they're just good at chatting, just like little Cyril who has plenty of siblings. If they're acting like a little shit then they're no different to little Dorothy who can be a little shit and she has siblings.
Onlies don't need socialising like puppies. When DD was 5 she would do Rainbows one night a week and swimming at the weekend. After a long day being bored rigid at school she might fancy going to the park or she might prefer to come home and chill in front of the telly. We'd play it by ear.
She is now a very gregarious 12 year old who can handle herself socially but - and this is a real life skill - she is happy in her own company and can occupy herself on YouTube for many an hour.
I would like to put this down to my superior parenting but actually I've spent a large part of her childhood ignoring her whilst I mess about on Mumsnet 
So what I'm trying to say - with lots of waffling - is try not to make your only your 'project' - just treat them the same way you would if they had siblings: indulge them and ignore them!