The old metric that tracks the growth of Twitter’s logged-in user base might not be relevant for much longer. “Viewing audience” might become an important thing to watch in Twitter’s performance numbers.

That’s because Twitter is getting much better at, and much more focused on, extending sponsored content out to other websites, where people who don’t even have a Twitter account might see it.

Take Twitter’s Amplify service. It’s all about taking an advertiser’s video and embedding it in desktop and mobile sites, with a “follow” button and Twitter stats and buttons beneath the video. (See the image below.)

The NFL might sponsor a video tweet, and Twitter would post the video (like a key replay from a game with a comment) on Twitter and on partner sites at a specific time during a game. This is good for the advertiser, and it might pump up viewership of the game.

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On mobile, this creates a “second screen” experience where the replay video is playing on a tablet or a phone in the living room while the game is playing on the TV.

Twitter AmplifyTwitter CEO Dick Costolo spoke repeatedly of off-site advertising in the company’s earnings call today. “Amplify allows us to partner with TV and other content creators and help them increase their audiences help drive tune-ins.”

Costolo says that people might see a key highlight video in a tweet and then get interested and turn on the TV to watch. That, he says, can increase viewership for the TV network, which in turn can increase the value of the ads around the shows.

It’s all about things taking place in real time, which is Twitter’s forte.

“Twitter has the unique ability to tie tweets and content to things people are talking about right now,” eMarketer analyst Debora Aho Williamson told VentureBeat. “That’s been a very valuable asset for Twitter.”

Advertisers like Facebook because of its larger reach, but the real-time nature of Twitter ads is something the microblogging site can do that its bigger rival can’t easily match, Williamson said.

And the off-Twitter video advertising is of great interest to marketers.

“I will be looking for the numbers of non-logged-in viewers Twitter has,” Shift CEO James Borow told me before Twitter earnings today. Shift is a social marketing software company, and a Twitter partner.

Borow is referring to the number of people who aren’t necessarily on Twitter but that see video tweets on other sites. “Everybody sees Facebook as Twitter’s main competitor, but we think Twitter is really becoming something more like YouTube,” Borow says.

Borow says his company and its big brand clients are most excited about Twitter’s capability to push branded video out to other partner sites, timed to key events.

And when you think about it, Twitter must do things like Amplify to keep growing and pleasing investors. As much media attention as Twitter gets, it’s still a niche site that’s only sees regular use from a relatively small number of people.

“Twitter is very useful for some people, but it doesn’t have the broad use case that Facebook has,” IDC analyst Scott Strawn tells VentureBeat.

“So Twitter has to find ways to move beyond that niche — to derive revenue from other forms of content like video.”

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