taking out the cost of the workforce (R&D, sales, associated GnA) working on revenue generation, ie. ads, analytics, etc. i'd suppose the rest of the costs of a typical Web2.0 site (like Twitter, FB, Reddit) comes down to basically hosting only and isn't that high - can probably be easily covered by donations. Or a real example - without meaningful monetization and thus related costs, WhatApp was fine with 55 employees serving $19B worth of user engagement.