Hopefully they WILL tell us about her life and not just continue to keep Frankie's existence both socially and family out of the lime light. I think the reason Frankie never had an imaginary friend is probably a difficult question to answer.
There's ALOT of possible reasons:
1. If the parents death theory holds true, emotional grief or trauma.
2. Stress from constantly working at her job (something tells me Madame Foster started her early).
3. Didn't feel lonely in a house of imaginary friends.
4. Wasn't bored enough to think one up in a house of imaginary friends.
5. Didn't want to make one in fear of it turning out like Mr. Herriman did (
joke)
6. Felt fine with her grand mother by her side.
I personally think the fact she grew up around so many imaginary friends is the real reason, that and the fact she has to take care of so many would help too. Whatever happened in Frankie's life is still a mystery, but we do know she's been to Foster's at least ONCE every year.
Judging by the photo of the year in "Big picture". The photos show her at a very young age so she ended up there alot, either constantly visiting or (again the possible parents death idea) lived there because she had no other relatives or elsewhere to go.
And if that is true it'd be kind of ironic when you think about it Frankie living at a foster home which is where she'd end up if had no relatives to take care of her. But anyways those are just my ideas and what not.