Before revision:  I show that the commonly-used linear-in-means  regression identifies a weighted sum of spillover effects with some negative weights, and characterize the estimand that is recovered by a simple difference in means in the presence of spillovers.
After revision:  I show that the commonly-used linear-in-means regression identifies a weighted sum of spillover effects with some negative weights, and that the difference in means in the presence of spillovers.