Old, good friends of ours did this with their daughter. She was 10, and went to France for six months. they were allowed one phone call once a week, no emails I don't think, but letters.
She had her birthday and Christmas with the French family.
It's done through an organistation, a sort of total immmersion learning the language thing.
My friend as three dd's, and this one is the least confident- she did it to help her confidence.
I don't actually think it's harmed her, but I'm not sure she's even more confident.,
The French girl who came back as the exchange counterpart was 9. My friend's family didn't like her .... they came to stay with us in the summer, and my heart went out to her.
They weren't overtly horrid or anything, but she obviously lacked family warmth, hugs and comfort. At night, she was tossing and turning, 'cos I went up to check on them, thought they were walking around, but it was her, obviously having vivid dreams.