Facebook thinks Amazon's ad business has officially become a threat

• Facebook listed Amazon as a competitor for the first time in its annual report.
• Facebook says it competes with Amazon in advertising.
• Amazon joins Google, YouTub, Tencent, and Apple on Facebook's official list of rivals.

Facebook has a new rival: Amazon.

The two companies have occupied the tech industry's top ranks for years, and they have increasingly encroached on each other's turf. But on Thursday, Amazon's fast-growing online advertising officially became a threat in the eyes of Facebook."We compete with Apple in messaging, Google and YouTube in advertising and video, Tencent in messaging and social media, and Amazon in advertising," Facebook said in its annual 10K filing with the Securities and Exchange Commission on Thursday.

The filing marks the first time the Amazon name has appeared in one of Facebook's regulatory filings, and it underscores the growing importance of Amazon in an online ad market long dominated by Google and Facebook.

While Amazon does not directly report its online ad sales, the company's "other" revenue, which is mostly comprised of ad sales, surged 95% year-over-year in the fourth quarter, to $3.4 billion.

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Amazon's share of the online digital ad market is expected to grow to 2.8% in 2019, up from 2.1% last year, according to eMarketer. Google is expected to claim 31.3% of the global digital ad market this year, while Facebook is expected to remain in second place with 20.5% share o the market.

The advertising business may be the most immediate clash between Facebook and Amazon, but it's unlikely to be the last. Facebook's video streaming service Watch is one of the company's most important initiatives, pitting Facebook against other entrenched video streaming players like YouTube, Netflix and Amazon Prime. And during the Facebook earnings call earlier this week, CEO Mark Zuckerberg cited a new effort to turn the Instagram photo-sharing app into a platform for e-commerce. If Zuckerberg has his way, Facebook's name will appear on Amazon's list of retail rivals someday, too.