1. The “Agency”: This company will provide an end-to-end service. They will manage your account, provide strategy, ongoing management and consulting for the initiative. This type of company believes its model is best suited for a client who believes the best work is done by experts. It is in their best interest to not mentor your internal resources (regardless of what they tell you). Use this option if you want to outsource your community or intend to get it going really quick or you believe you dont have time (maybe because your competition has beaten you to it) to do it yourself. Most PR agencies of yesterday now offer social media as an option in this list of services.
2. The “Specialists”: This type of company is good in one or more facets of community development. For example they are good at “developing your alpha members” or “viral marketing” of your community or “moderating” your user generated content. If you have one or more members of your team that have some experience with building a community and you only need additional “hands on deck” to complement a specific area of your exposure, this is the best type of company for you.
3. The boutique or “one woman bands”: These companies / individuals are best suited for very large companies who need deep expertise in an area or the very small (community is very small so we can have one person do it all). If you are a mid sized company, this is not the most suited option. This company is obviously risky since everything depends on one person.
Update: My friend Tac talks about Marketing vs. PR agencies.