Catholics can use Facebook to help distribute news, images, videos, and create discussions among members of the faithful. It is a little harder to evangelize to outsiders on Facebook, but it is possible.
Facebook boasts more visitors every day than any other website in North America. This is reason enough for every Catholic parish and organization to, at a minimum, keep tabs on what's happening on Facebook.
Parishes and organizations that maintain Facebook Fan Pages and Groups, and actively promote the use of Facebook as a collaborative tool, have much to gain. Already many of the people whom parishes and organizations are trying to reach are present on Facebook. The key is to providing (a) information about your organization, and (b) a channel through which people can leave comments or send you feedback.
Facebook allows anyone to create a group or fan page, and on these pages, you can tie in your website's news (RSS) feeds, Twitter accounts, etc. fairly easily.
The pages underneath this one will give instruction and best practices for these different avenues of communication.