Google dominates search, but also dominates e-mail, mobile devices (less so, but still a major player), and is heavily involved in many other tech verticals. That they dominate isn't the problem.

That they are leveraging their hegemony across all of these verticals to drive their main source of revenue, advertising, is where the concern comes. The fact is it is nearly impossible for a non-entrenched player to enter into the market without either being drowned our or bought-up by one of the big players is where the monopolistic concerns arise.