As Facebook has Messenger, Steam has Steam Chat.
Steam Chat

Valve just released the new Steam Chat application for both iOS and Android mobile devices. This is a new stand-alone app that allows you to have all of the features of Steam chat in a more lightweight application.

If you're still confused, maybe this will make it a bit easier to understand. You know how there is the bloated Facebook app for mobile phones? And you know how Facebook has a private messaging system that they broke off into its own app called Messenger? Messenger allows you to chat to your Facebook friends without the bloat of actually having to use the Facebook app.

It's the same idea here for the Steam Chat app. It's all chat without stuff like the Steam Store and marketplace. Valve says that this app will give you the features of the desktop client but on your mobile phone.

In addition, they say that there are more features coming soon. These features include things like voice chat. You can always share your thoughts on this new app by participating in the Steam Chat App group on Steam.

You can download Steam Chat for iOS and Android.

What if you already have the normal Steam app installed? Valve says that they will see "significant upgrades" made to that app that will focus on account security. They have plans to include "better Steam Guard options to help securely log into your Steam account, such as QR codes and one-touch login, and improved app navigation." My guess is that they're going to completely separate chat from the old app and use the old app mainly for Steam Guard and possibly Store and library functionality.

The app includes many key features of the Steam Client desktop chat, including:
  • Friends List - See who’s in game or online at a glance. Never miss an opportunity to play.
  • Rich Chat - Your chats get even better with higher fidelity links, videos, tweets, GIFs, Giphy, Steam emoticons, and more.
  • Invite Links - Add new friends on Steam with a link. Generate an invite link you can text or email.
  • Customizable Notifications - Mobile notifications mean you'll never miss a message or game invite. You can customize your notifications per friend, group chat, and chat channel.
  • Group Chats - Get everyone on the same page. Groups make it easier to do things like stay in touch with your communities and organize game night with your best friends.