So if you were looking at the market a few years ago through the eyes of an E.U. antitrust regulator, you might have anticipated some trouble for Facebook on Android phones, and you might even have ventured that Google Plus would become a hit. After all, here was a small, late-moving upstart pitted against a bundle-friendly giant that owned the operating system.

Of course, that’s not how things turned out. Google Plus is now all but dead. And Facebook did not just survive the shift to mobile phones, it became the unstoppable force in the industry.

Facebook now derives the vast majority of its revenue from mobile ads. Its sales are growing at a breathtaking clip. People now spend more time on the Facebook app than doing anything else on their phones. Facebook’s embedded web browser — which opens if you click a web link from within the Facebook app — is now used to access nearly as many web pages as the included web browsers on Android and iOS, according to a report by the research firm ScientiaMobile.

How did Facebook get so popular on phones? On the whole, it happened the old-fashioned way: People loved Facebook, they had no trouble finding it in mobile app stores, and they began to download and use it. In other words, unlike in the desktop era ruled by Microsoft, Google’s platform dominance on mobile phones does not appear to have posed any hindrance to a competitor’s rise.

The same appears to be happening with other Google rivals. Google bundles its cloud storage system, Drive, with Android, but that hasn’t stopped Dropbox from gaining 500 million downloads on Android phones. Google bundles its music service with Android, but with 30 million paying subscribers, it is Spotify, not Google, that is the dominant force in streaming music. (Google has not disclosed its subscriber numbers.)

The E.U.’s objections go beyond the supposed restrictions that Google places on Android’s app ecosystem. Another key charge involves Google’s dominance in web search. Regulators say Google ensures that it remains Android’s dominant search engine both by requiring phone makers to install the Chrome browser on their phones and, in some instances, by paying manufacturers to position the Google Search app on the phone’s home screen.

Mr. First, the law professor, said the search issue isn’t as clear-cut as the question of whether Google has restricted rival apps. But he noted that the general flexibility of Android — the fact that, in general, Google makes it pretty easy for device makers and users to tinker with the software — cuts in Google’s favor.