With the advent of a range of social networks, librarians all over the world have started to ask themselves how this new technology can be used as a way to market their libraries. Today's information websites and search engines are difficult competitors for libraries and information centers, and are forcing traditional information institutions to market themselves in new ways. Marketing theories have traditionally concerned themselves with money transactions and marketing of specific services and/or products. Relationship marketing theories, on the other hand, focus more on the relationship between the customer and the company that produces the product or service. These two parties are seen as co-producers, and relationship marketing theories tend to focus more on a win-win situation, than on the company gaining power of the customer. The relationship is the important thing, and the theories concentrate to a great extent on how to nourish these relationships. This paper is a discussion about whether Facebook as a social network is a usable tool for library marketing. Since both libraries and social networks are all about relationships, we chose to make use of a relationship marketing theory to investigate this. This theory was Evert Gummesson's theory about the most important relationships in marketing. The literature we have used in our analysis shows different meanings when it comes to how to best, if at all, market a library on Facebook. The conclusion is that there is not one correct answer and every different library has to explore what is best for them.