There is no simple answer to this question. It depends on a variety of factors, including the purpose of the group, the demographics of the group members, and the nature of your product or service.
That said, in general, Facebook groups can be a powerful tool for marketing if used correctly. They provide an opportunity to connect with potential customers in a more informal setting and build relationships with them. You can also use groups to share valuable content and drive traffic to your website or other marketing channels.
The key is to make sure that you provide value for group members and don't simply spam them with advertising messages. Be helpful, engaging, and responsive, and you should see good results.