Facebook is "testing changes to the multi-friend selector to make it easier for users to invite friends who don't have a Facebook account. It prompts you to select Facebook friends, and then to import the email addresses of your friends who don't have Facebook. That means your friends might be inviting you to all kinds of stupid stuff via email, even if you're not on Facebook.
The new contact import feature can get email addresses from webmail clients like Gmail, Yahoo! Mail, and mail apps like Outlook and Apple Mail. It will also be able to import other types of contacts, like CSV files, eventually. Once imported, these email contacts can be selected in invites just like Facebook contacts," writes Facebook developers blog.