Zillow's stock plummeted 6.64% in pre-market trading on Monday, following reports that Google is testing a real estate ad unit portal that could directly compete with Zillow's core business model.

The test, currently live in limited markets and only on mobile, includes features like full property detail pages, tour requests, and agent contact links—functionalities central to Zillow's platform. This development has raised concerns among investors about potential increased competition in the online real estate space.