By the time the Super Bowl goes to its first commercial break on Sunday, Feb. 1, a lot of people will know what to expect. They will have watched previews seeded around the Web and aggregated into Super Bowl ad teaser posts.

That’s because today, it has practically become standard operating procedure for brands to tease or “leak” a Super Bowl spot days, or even weeks before it actually airs to gain additional viewers on top of the estimated 110 million who will watch the game on Super Bowl Sunday.

That's the dirty little secret of "leaked" Super Bowl ads: They are never leaked but carefully placed as part of a strategy to amortize the cost of an ad, which is $4.5 million for 30 seconds of time on NBC and millions more to produce. And while brands would have you believe these ads are so loved they spread on their own, they're actually backed with millions of dollars in paid promotion to surface on YouTube and other websites. But as audiences wise up to the tactic, it's getting more expensive and less effective.

According to Web audience measurement firm Visible Measures, 75 percent of Super Bowl advertisers put up some sort of preview Web content before the big game last year. This year, with the game still 10 days away, advertisers including Anheuser-Busch (which owns Bud Light), Mars (which owns Skittles and Snickers), Doritos and Toyota (which owns Lexus) have already gotten started.

That’s a big jump from 2010, when E*Trade and Volkswagen became the first brands to put up Web content previewing the Super Bowl ads that were scheduled to run. Those campaigns performed incredibly well, with postgame views that outdid the rest of the Super Bowl ad field by almost 700 percent, according to an analysis from Visible Measures.

Competitors followed their lead, and by 2014, with most of the field employing the same tactics, the returns on that strategy had diminished. Last year, ads paired with Web content released ahead of time were, on average, watched two and a half times more online than those that weren't.

That lift may not last, either. According to a separate research study conducted by the video advertising technology company Unruly, social media shares of Super Bowl commercials slipped last year for the first time, declining 29 percent from the previous year even as overall social sharing of video online leaped 22 percent.

“You have to earn shares,” said Devra Prywes, Unruly’s vice president of insights. “Last year’s ads didn’t do that.”

But brands weren’t earning shares by simply releasing their content quietly and crossing their fingers. They were – and are – buying large chunks of the views, at no insignificant cost, both in the run-up to the Super Bowl and in the days following it.

“We recommend a minimum threshold of spending before the game and after the game,” said Seraj Bharwani, Visible Measures’ vice president of analytics.

Bharwani did not discuss specific campaigns, as Visible Measures employees are not allowed to discuss the amounts its clients spend on advertising. But buying enough viewers to rank among the Super Bowl’s most-viewed ads – say 18 million to 20 million views – can cost a significant amount.

“If you paid for all of it,” Bharwani said, “you're talking about a one-and-a-half-million-dollar investment.”

The hope, of course, is that the content catches fire among a small percentage of those viewers, and organic shares propel the brand’s message far and wide. But viral is not the same thing as free. “The term viral connotes a lot of myth,” Unruly’s Prywes said. “You need many to infect many.”

That means spending, and spending fast. According to research by Unruly, the window of time in which people share video content has gotten a lot smaller. Today, 42 percent of the shares a Web video gets happen within the first three days, up from 25 percent a year ago.

Because there is such a short window to work in, some Super Bowl brands would rather not jostle and compete with one another. They'd rather get one peak of attention early, then ramp up for another one after the game. But with audiences losing interest, and more brands jumping into the fray, that could push costs even higher on what is already the world's most expensive kind of advertisement.

"At some point," said David Campanelli, a senior VP at agency Horizon Media, "it stops being cost-effective."