I am not going to tell you the answer to the question in the headline. The purpose of this post is to get you to think about this question, and the many variations of it with other media channels, because it is the work to get to the answer more than the answer itself that will provide the biggest payoff for your events.
As a media strategist and buyer who has placed millions of dollars in advertising for my clients' shows, I have spent a lot of time dealing with these type of cross-media scenarios, so I share with you some of the variables you would need to consider for this specific scenario of standard web retargeting cookies vs. Facebook likes.
• Creative Assets: How will the different ad formats affect your ability to achieve your objectives? Standard web retargeting enables you to use standard IAB units, i.e. 300x250, whereas advertising to fans on Facebook does not.
• Costs: What is the cost per acquisition (CPA) for you to acquire cookies vs. likes, and what are the costs (CPM/CPC) to deploy advertising against these cookies and likes?
• Time Decay: How will the age of the cookies and likes affect your results? Do you think a like you acquired when you first launched your Facebook page a few years ago is as good as a cookie you placed yesterday? Likewise, what is the value of cookies that are much older than your likes?
• Purchase Funnel: While cookies will be more effective at lower funnel and more transactional goals, they don't offer the social layer that likes do, which can help in upper-funnel branding and awareness. Which is more important to you for the task at hand?
• Scale: Will you be able to achieve more scale against your target with cookies or likes?