Alphabet reveals YouTube revenue for first time

Google parent company Alphabet on Monday reported its worst fourth-quarter revenue growth in four years, missing analysts' estimates during a period which saw its main advertising rivals beat expectations.

The company has been the web's biggest draw for advertisers for a decade, helping it last month to become the fourth public company to top $1 trillion in market cap.

But new concerns have emerged among investors about whether its dominance will last, as U.S. antitrust regulators investigate Google….and as Amazon and Facebook continue to grow their ad businesses globally.Google over the last year has posted slowing ad sales growth. It has blamed foreign exchange rates, among other issues.Expenses have also ballooned with the hiring of thousands of salespeople, building of new data centers and marketing efforts.Monday's earnings report was the first under Chief Executive Sundar Pichai - also the Google CEO - who gained the additional role of Chief Executive of the parent company after cofounders Larry Page and Sergey Brin stepped back further from day-to-day management.Pichai's new role brought with it changes to Alphabet's financial disclosures, which investors before generally criticized as too opaque ... specifically, how much ad revenue its video platform YouTube makes.

In a first, Alphabet reported that number - it was over $4.7 billion for the fourth quarter, a 31% increase year over year according to Variety.

Shares of the company fell in after-hours trading.