Messaging rules the app stores these days... as does Facebook. Here is a screenshot of the Google Play Store's Top Free Apps: #1. Facebook Messenger #2. Facebook #4. Instagram (Facebook) #5. Snapchat ... #9. Skype #10. Kik #13. Twitter #16. WhatsApp
Notable others: music has #3 and #11 with Pandora and Spotify, respectively. King has games at #8 and #12 with Candy Crush and Bubble Witch 2. And Netflix is #7.