From the perspective of web users, there is choice(even if heavily skewed by default search in browsers). But from the perspective of web sites/web companies/web devs/web advertisers, there is very less choice and bordering a monopoly, as this example illustrates. Even worse if you're someone like MapQuest, a shopping site, or Yahoo Finance, since most people tend to search locations on search engines/browser and are quickly steered to their equivalent competitors from the search engine company. Anyway, usually antitrust regulations are about market power and influence, for example see DoJ vs. Apple over eBooks, even though iOS never even had majority share, forget about a monopoly.