I think that could be interesting. As others have said, whatever the computations are, if they are being done anyway, might as well use the generated heat for something useful instead of letting it go to waste.

But I wonder what would happen with such a system in the summer? For example in most on France it gets pretty cold for long enough every year that having a proper heating system and good insulation makes financial sense. But during the summer it's pretty hot, especially in cities. While the heating can be turned off between March and October (give or take), Facebook & co would probably like their DC to keep on working, so to keep on heating, year round.