Here's the thing about rumors: the ones that spread the fastest are the ones that people reeeeeeeeeally want to believe.
Fans want Marvel to take control of "Fantastic Four." It's that simple.
Fox isn't planning to give it back to Marvel, though. Whether anyone wants to believe it at this point or not, they still have plans for the property. What form those plans will take is still under discussion, but they haven't just given up and thrown in the towel on it. They took a critical beating on the film, they had big problems with the press and with their director, and it's been a financial disappointment, to say the least. Even so, they managed to squeeze about $165 million out of the movie when you factor in the worldwide box-office.