Skype (or should we say, Microsoft) is only the fourth company to meet this threshold. Google obviously has the most, with more than a dozen apps boasting over 500 million installs. Facebook gets the number two position thanks to its popular social network, Messenger, and WhatsApp. Samsung gets number three, but since its sole app in the rankings is the Samsung Push Service, that's mainly a technicality. Skype really has third place bragging rights here.
Considering how deeply entrenched Skype has become (I've used it to catch up with friends and apply for jobs), it's not a surprise to see it get this far. Take a bow, Skype.