The biggest app advertiser on Facebook is Instagram, a subsidiary, according to a data firm that tracks mobile campaigns.
Instagram represents up to 5% of app-install ads on its parent social network, Sensor Tower said in its latest report, which tallied app activity on millions of devices. It has typically been among the top 10 app advertisers on Facebook but jumped into No. 1 in the third quarter of this year.
"They seem to always have campaigns running to drive new users to the app," said Wes McCabe, head of product marketing at Sensor Tower.
Instagram has grown to 500 million monthly users today from 100 million monthly users in 2013, likely aided in part by its promotion on Facebook.
But social-media competitor Twitter, whose 317 million monthy users are growing only slowly, is barred from advertising on Facebook.
That's because Facebook, a digital player so powerful that any changes to what it shows or doesn't show users can have huge effects, has a policy that restricts rivals' advertising.
Twitter has tried to run app-install campaigns through Facebook but has been denied, according to an advertising executive close to both Twitter and Facebook.
"This is a huge challenge for any app advertiser that Facebook considers a competitor," the ad exec said, speaking on condition of anonymity to avoid antagonizing Facebook.
Twitter declined comment.
A spokesman for Facebook declined to comment but pointed to a clause in its policies reserving "the right to reject, approve or remove any ad for any reason, in our sole discretion, including ads that negatively affect our relationship with our users or that promote content, services, or activities, contrary to our competitive position, interests, or advertising philosophy."
Facebook's policy is not unusual. Most publishers and media companies have policies that restrict their rivals from buying ads. NBC doesn't run commercials for CBS programs, for example.
But Facebook's dominance online has no parallel in traditional media. It has become a dominant force in the mobile app ad market, and is often the main source of growth for startups and developers, which pour money into the platform to build their audiences.
"Facebook is the biggest major player, and probably more than half the market," said Alexey Malafeev, Sensor Tower's co-founder.
EMarketer has estimated that U.S. ad spending to promote apps will reach $5.7 billion this year.
Facebook does not break out how much of its ad revenue comes from app-install campaigns, but most analysts say it is a substantial piece of its business.
Facebook does face growing competition for developer dollars with Google expanding its app ads product, Apple introducing paid search ads in its App Store, and startups such as Snapchat and Pinterest offering app-install ads as well.
Twitter also sells app-install ads, and it is even open to Facebook and Instagram if they wanted to buy there, according to a person familiar with Twitter's policies.
Like Facebook and its Instagram subsidiary, Google divisions are Google's own biggest app advertisers, Sensor Tower said. Google App and YouTube Music were the most marketed apps on its platform last quarter.
Not all rivals want to spend on each other's platforms, partly because that could expose information about their results and consumers to the competition. And not all social media apps run install campaigns.
White-hot Snapchat, for example, doesn't appear to have run any such ads, according to Sensor Tower. Snapchat declined comment for this story.
Snapchat, Instagram and Twitter are in tight competition for users. In 2012, Facebook bought Instagram to stake a claim on the mobile future.
Twitter has wanted to run app-install campaigns on Facebook even if that would mean that its rival got a glimpse at any customers it acquired and other data, according to the agency executive close to both companies.
"The upside is huge if Facebook lets you advertise," the agency exec said. "Most competitors would be very happy to be on there because it's a net win if you grow your users and Facebook gets some data."