It's an interesting question though, because it depends on which 'market' you're talking about. This is a vexed question in antitrust law [1]. There is some pending litigation in the United States as to whether Apple illegally monopolized the market for iPhone apps [2], but it's been brought by app purchasers rather than the regulator, and Apple has taken the case to the Supreme Court to argue that the purchasers don't have standing to sue.
[1]: https://en.wikipedia.org/wiki/Relevant_market
[2]: http://www.scotusblog.com/case-files/cases/apple-v-pepper/