I'm highly skeptical that putting the extremes in greater contact would cause de-radicalization. I think it's more likely that would cause the extremes to further polarize and dig in for apocalyptic battle for the fate of the world.
I think a more-likely "de-radicalization" path is to hyper-amplify the center while suppressing the extremes. Specifically exposing users to a lot of content one "notch" toward the center and some content two "notches" in that direction (e.g. extreme right user gets a lot of moderate right and some centrist; a moderate left user gets a lot of centrist and some moderate right).