A standalone Steam Chat app is now available for Android and iOS devices. It features most of the functionality of the desktop version, including a friends list, game invite links, group chats, and "rich chat" – basically chat riddled with gifs and expanded links.
In its announcement, Valve wrote that it intends to keep improving the app with the forthcoming roll-out of voice chat. "With Steam Chat moving to its own dedicated app, the original Steam Mobile app will see significant upgrades focused on account security. Our plans 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."