The Planetary Extractor (Synthetic Race) offers an adjacency level bonus to Social Construction. I first stumbled upon it when I realized that adjecent factories didn't receive any bonus at all. I thought it might be by design, but eventually I realized there are no buildings of type Social Construction in the game. At least none available to the Synthetic Race.
Therefore I consider this a bug.
Since the building itself is of the All Construction type, it would make sense for it to offer an adjacency bonus of that type. Maybe even to Ship Construction as well - there are so few ship construction buildings, it would make sense that at least this central building would be able to boost these as well.
I'm referring to the current base game including Crusade.