A monopoly should be defined by barriers to entry, not just market share. There's nothing preventing other large companies from entering the search space, and in fact many of them have (yahoo, microsoft, etc). There are true monopolistic firms like Comcast or Time Warner, where there are serious logistic/economic barriers preventing other firms from entering the market. This doesn't seem like one of them to me.
There is tremendous barrier to entry in the search space - otherwise Bing wouldn't still suck compared to Google. The point of competition is to get good enough to steal market share from the entrenched incumbents or gain new users somehow. If that doesn't happen, for whatever reason, then you don't have strong competition.