Most companies and organizations would do well to establish a presence on both. If you have a global audience, you should definitely be on MySpace. Facebook is more local. Expect to reach niche audiences with both approaches, as part of an integrated online marketing strategy.
Check out this video in which some top marketers address the importance of participating on these social networks: http://link.brightcove.com/services/link/bcpid2891008001/bctid2953456001