How is search a network effects business? It seems to me that each additional user would contribute less than the previous one, as opposed to Metcalfe's law (where each additional user adds more value than the last). How would paying to be a default be a network effect? Perhaps the capital cost to competing with Google is high, but if it were worth it, there are a number of firms which could afford it; it seems like it's just not worth it.

You can determine if a search result is effective for that query if a user stops searching after clicking the link. But only if you have enough users for any given query.

Also ad networks

Maybe network effect isn't the right way to describe it but clearly Google has a ton of inertia here since their company is literally the verb for search.