Some advertisers rejoiced Tuesday when Facebook Inc. introduced long-awaited video advertisements. A bigger question is how users will react.

Marketers "have to be sensitive," said Tony Pace, chief marketing officer for the Subway sandwich chain, which advertises on Facebook. "If someone said [this video ad] is going to run whether consumers want it or not, that would give me pause," he said.

Facebook said its first video ad, a teaser for the coming sci-fi film "Divergent," would begin appearing Thursday, marking an effort by the world's largest social network to grab a slice of the $66 billion annual U.S. TV advertising pie.

The video ads, which the company says are still being tested to a limited number of users, will start playing automatically as users scroll through their news feed, the central real estate in Facebook's desktop and mobile platforms. They will initially play without sound; users can stop the ad by scrolling past it in the news feed.

Some users are wary. "This may be the proverbial straw that breaks the camel's back," said Lee Kirschbrown, a senior citizen in Tampa, Fla., who is irked by existing Facebook ads. Video ads "will only make me madder," he said.