Announcements touting pacts between agencies and social-media companies are plenty fashionable. But how significant are they? Many are thin on details, making them look like nothing more than a public high-five.

"There are a lot of kind-of-empty announcements [coming] out of the marketplace," said John McCarus, senior VP at DigitasLBi. "The truth is, as long as the media keeps printing them, there will continue to be announcements."

For example, last month Instagram and Omnicom announced the holding company would spend as much as $100 million over the course of a year with the photo-sharing site. But what special treatment Omnicom was getting wasn't spelled out.

Starcom MediaVest Group's pact with Twitter unveiled a year ago was even more nebulous in its financial details -- hundreds of millions of dollars were to be spent over a "multiyear" period. The agreement included access to premium inventory like promoted trends, Twitter-data integrations and preferred pricing. However, SMG didn't make spending commitments that would result in a penalty if unmet, according to Lisa Weinstein, president-global digital data and analytics at the agency.

Six weeks after that deal, WPP and Twitter entered into an agreement centered on data. No financial details have been disclosed though media-buying arm GroupM was party to it, suggesting ad dollars were involved. The deal has led to Twitter and Kantar creating social TV-ratings for markets outside the U.S., however.

For Twitter and Instagram there's an obvious benefit: The linkups give the impression their ad models are solid and more revenue is on the way. But often the upside for agencies in these deals can be harder to see. One benefit is that they allow media agencies to cast themselves as "innovative" in digital. Another is being able to collaborate with the publisher on product development.

Starcom MediaVest Group's Ms. Weinstein said the deal created a "social-TV lab" powered by a Twitter-data integration, allowing 15 clients to access research around tentpole TV events to see how their Twitter campaigns fared in terms of metrics like brand consideration and purchases. "Being first means you're exclusive certainly for a period of time," she said.

In terms of product development, Ms. Weinstein said Twitter added the ability to exclude terms from its keyword-targeting ad product on the basis of SMG feedback and that the deal was a springboard for a closer working relationship. OMD Chief Digital Officer Ben Winkler -- whose agency repped two of the first brands to advertise on Instagram -- said collaboration on product development was key. "Without this deal, the relationship is very straightforward: Instagram sells, OMD buys," he said. "That doesn't leave a lot of room for developing ad products that clients deserve."

Jonathan Schaaf, Omnicom Media Group's president of U.S. digital investments, said that Instagram has created a dedicated team of employees spanning strategy, sales, operations and product for Omnicom's agencies. "It's not just a sales rep closing insertion orders," he said. Instagram staffers do treat Omnicom work as a high priority but they aren't entirely limited to Omnicom work, according to another person familiar with the pact.

Omnicom shops will also be able to use otherwise unavailable Instagram data surrounding user behaviors and connections on the photo- and video-sharing service, Mr. Winkler said.