I didn't have imaginary friends so much, but I did have a whole cast of imaginary characters of endless stories. Two or three of them, in fact. I still do – whole series of novels that will probably never be written.
But I used to pretend to be different people. It started with Mowgli and went on from there.
When other little girls were playing house, my best friend and I were playing Medieval Ireland or riding golden horses or diamond Chevrolets – I distinctly remember my pal inventing that diamond Chevrolet, and the little flicker of surprise when she mentioned it – it hadn't occurred to me that Chevrolets were "allowed" in the particular fairyland we were visiting that day.
My son, when he was about two, had a whole retinue of imaginary frogs that followed him around – until the day he laid about him with a screwdriver and slaughtered them all!
My daughter has a whole clan of invisible friends – like Sirona, I'm not sure all of them are imaginary. She says they live in her head and sometimes one of them will be her for a day or so. They seem to be a pretty level-headed lot.