To be brief, more data (generated by clicks and queries) leads to better ranking. Better ranking attracts more users. Starting a search engine from scratch without any user data is a big disadvantage.
Basically, you're benefitting from other people who are searching for the same thing as you on the same search engine. (I'm sure that's not the only network effect, but it's one of them.)
Well, first of all, this isn't true. Google and FB aren't so profitable because of a distribution monopoly. They're profitable due to network effects.