Facebook is trying once again to catch Snapchat's wave. The social-networking behemoth is launching on Tuesday a new app called Slingshot that allows people to share short-lived photos and videos with one another. This concept, which caught fire through competitors like Snapchat, encourages people to share less carefully edited photos and videos by promising they'll be deleted shortly after they've been seen. One twist, though, is that unlike most other apps, Slingshot requires users to share something before they can view any images from others. That twist -- along with a focus on sending to groups, rather than to individuals -- is what Facebook hopes will broaden Slingshot's appeal.