> consumers overwhelmingly choosing a product

By "product", I take it you mean their advertising services? Most of the other products are not paid for. In fact, critics are often fond of pointing out that the consumer is the product in this case.

Whether this action is right or not, parallels can be drawn with the Microsoft investigation: Microsoft allegedly used its position within the desktop to "push" their browser; Google is allegedly using its position within search to "push" their browser.

On a personal note, I'm not convinced that this is grounds enough for antitrust action. There could be a personal data related angle to this, but then again that would not come under antitrust.