Just saying Google is dominant in search isn't enough of a reason to prosecute, but EU privacy rules that are so widely feared by IT people in the U.S. would be plenty of reason to both investigate and prosecute.
A micro-monopolism trial featuring Google in an EU court would hardly be worth watching, especially with the time difference -- like the Winter Olympics without American skating stars or skimpy outfits.
A privacy trial for Google and other U.S. services like Facebook would not only be a lot more entertaining, it would pathfind for a lot of U.S. companies wondering if they're ever going to be able to move a bit of data anywhere inside the EU for fear of running afoul of privacy and data-location rules.
The EC is lightening up those rules a bit, or is at least talking about it. It hasn't happened yet, though, and the toughest data protection rules will take effect next year. They'll replace rules put in place in 1995.
Without someone to go in and not only test those rules, but map out the enforcement procedures by actually being prosecuted, the rest of global IT will be in the dark about how big the threat is and how far the EC is willing to take it.