You'd have a network effect if the presence of third parties (the "network") coerces you to choose one product over another. Say, Facebook over Diaspora because

You'd have a network effect if the presence of third parties (the "network") coerces you to choose one product over another. Say, Facebook over Diaspora because your peers are all in the former but not the latter (ignoring the other, quite material, differences for this example), or the monopolist's telephone network over another (that the monopolist refuses to peer with at reasonable rates). Possibly ad-networks, too (tons of ad spaces spread over the entire web with good payout rates) - which is why the FTC requested some changes to the AdSense ToS (regarding the use of multiple ad networks) and got them. IMHO the attempt (by several european telcos) to force open messenger networks for interoperability might get more traction than attacking search engines for "network effects". I just hope that (should this ever happen) it will be a poisoned gift in that instead of giving telcos preferential treatment, they (Facebook/WhatsApp, Google, MS) simply provide fully open interfaces - and users continue to route around the failure that is telco messaging.