Yep, but all else being equal you would also choose the one with fewer other customers. So in terms of the first-order effects, each additional user (as in, rid

Yep, but all else being equal you would also choose the one with fewer other customers. So in terms of the first-order effects, each additional user (as in, rider not driver) actually makes things worse for existing users! There's the secondary effect that each additional user encourages more drivers to sign up, but while this might compensate for the first-order effect it's not going to overcompensate. For another perspective, you can look at things empirically. Has Uber used network effect to dominate the market and force out competitors? No; Lyft, traditional taxis, and foreign ride-sharing companies are still around. To the extent that Uber is winning, it's because it has a better product (for a definition of better that includes burning VC money to lower prices). Contrast this with Facebook: the only major similar product in recent history is Google+, which died pretty quickly. All other successful social networks differentiate themselves in some way, because they have to.