The instant messaging app imo has crossed the threshold of a half-billion Play Store installs, and there's a pretty good chance you've never heard of it.
imo was launched by former Googlers way back in 2010, and its claim to fame was offering an instant messaging platform that gathered all of your other messaging accounts in one place. That included AIM (RIP), Skype, Facebook, Google Talk, and yes, even MySpace.
But with the rise of challengers like WhatsApp, Skype pulling its access, and the "technical headaches" of supporting all these disparate services, imo ended its support for third-party platforms in 2014. As reported by Harry McCracken at the time, imo trudged on ahead to focus on its own network for text-based chatting and video calls.
Though it seemed to have completely dropped off the radar, the service has been growing steadily for years. According to AndroidRank, the imo Android app does particularly well in countries such as Singapore, India, Sweden, Russia, and ranks number 2 in the communications category in Bangladesh.
Not many apps reach 500 million installs, so this is a particularly remarkable feat for a service that many had all but forgotten about.