A large number of people are using Whatsapp on their phones, but a very good alternative for that app is Telegram. The main difference is that the Telegram devs have also released a desktop client for the Linux platform, that is also open source.
It might not seem like much, to have a desktop client for a messaging app, but we would all love to have something from Whatsapp. On the other hand, Telegram is a less known app, but it's just as good (if not better some might say). Now, less known still means it has dozens of millions of downloads for the mobile platform and that is a lot.