Out of fear that Steam would take over my life more than it already has, I never bothered to grab the Steam Mobile app. Valve has updated the iOS version with added functionality and the more I read about it, the harder it is to resist downloading. Good thing my phone isn't within reach.
As of v1.3, Steam Mobile now supports the sending and receiving of messages to offline users, and the option to view your chat history. The storefront has also undergone design changes, and the user interface for friend requests and group invites has been streamlined. It's also now possible to see which platform (mobile, desktop, or Big Picture) a friend is on.
Lastly, the app has push notifications for offline messages and friend/group invites, so be sure to go to the Steam Preferences page in the settings to adjust which notifications you'd like to see.
Steam Mobile Updates [Steam]