The Replize tab manages all replies sent to you by other members. In order to send something to another member, simply include @username in the comments field and it will get sent to that user instantly. Every reply is highlighted in the user's homepage news feed. You can choose to be notifed via email if that's what you want. You can also just click the reply link next to a comment and their username will be auto-populated.
You can reply to several members at one time by adding all of ther usernames, i.e. @JohnDoe, @GIJane, @Rizzn, etc. In addition, soon there will be an auto-complete feature for any @username you are following or vice versa. This will ultimately allow users to send replize as tweets on twitter.
The other new feature announced is the ability for users to connect their Flickr account to socialmedian. Obviously, this will be beneficial for mobile users who want to upload their photos and use them within socialmedian. When connecting your Flickr account you will have the option to add all of your photos and favorites or filter them according to keywords (tags).