  We introduce a measure for the level of stability against coalitional
deviations, called \emph{stability scores}, which generalizes widely used
notions of stability in non-cooperative games. We use the proposed measure to
compare various Nash equilibria in congestion games, and to quantify the effect
of game parameters on coalitional stability. For our main results, we apply
stability scores to analyze and compare the Generalized Second Price (GSP) and
Vickrey-Clarke-Groves (VCG) ad auctions. We show that while a central result of
the ad auctions literature is that the GSP and VCG auctions implement the same
outcome in one of the equilibria of GSP, the GSP outcome is far more stable.
Finally, a modified version of VCG is introduced, which is group
strategy-proof, and thereby achieves the highest possible stability score.
