The essential need is to do both - if you hear advice to the contrary, the people you are talking to really don't know what they are talking about.
Think about a bike wheel - the built community is your hub - you control it and host the conversation inviting the world into your party, it powers what your business does...the affiliated communities (and there potentially many - see our social media octopus) are the spokes - extending your good stuff into the microcosms of the web and getting your content unchained, loose and fancy free.
Here's the key reason why you need the hub and spokes to your community bike - if you look at the 6 key requirements of building effective community (there are a few others - but these are the day to day operating essentials), a built community (i.e. Dell Ideastorm) vs. an affiliated community (i.e. a user forum or a Facebook page) perform much differently on these steps.
Outreach - for most prospective communities - the people you need to find and recruit are "out there" not "in here" - last time I checked finding 1.1 billion people in social networks, 350 million people on Facebook, 50 million LinkedIners and 20 million early adopting Twitterites then dropping a phone call to your very best friends - Advantage- Affiliated Communities
Seeding - identifying the right group of users to be your front row of testers, collaborators, users and ambassadors - takes a mix of scale - having a lot of people enter the candidate funnel and knowing what they are doing and saying in the world out there but on the other side, also understanding intimately the people that are already related to you and their intensity of commitment and interest - Advantage - A Tie
Engagement - competing with 150,000 Facebook pages and 100,000 iPhone apps are way too difficult, to create any type of meaningful engagement, the user experience in these arenas is to graze and sip not to settle into one space and go deep - on top of that, all of the meaningful metrics, insights and opportunity to build a relationship belong to the platform not the sponsoring party - you are at their whim - finally, any customization requirements are the mercy of what the social network platform or user forum allows you to do...it simply pays to build your own community to engage people on a deeper basis Advantage - Built Communities
Collaboration - oftentimes in a community you want to hive off a discussion, a project, an idea, a group and get a subset of engaged and informed people to rally around it away from your general mainstream group - once again, tough as nails to create that interaction on the popular social networks - you are just another subway stop on a user's very long train tracks of their web social and content life - creating this sense of commitment, affinity and collective interest happens in built communities enhanced by the ability to control and design your world your way consistent with community user's core needs Advantage - Built Communities
Affiliation - people join and buy into communities for three reasons - intrinsically - they buy into the values of what you're doing, extrinsically - they will appear better to their peers and explicitly - they get something for their time and effort - whereas built communities may reinforce a sense of intrinsic "we're all in this together" value better, affiliated communities allow for an expanded broadcast of your reputation driving extrinsic value - then it's a battle between the built communities ability to reward anybody at any time more easily vs. affiliated communities ease of effort and ability to recruit and be noticed be others doing good stuff Advantage - Tie
Rebroadcast - good content usually rises to level of its quality - and the good stuff usually goes "viral" in social networks and user forums not within the confines of the built community itself, check out the New York Times as well as most established media and grassroots blogs and the majority of their web traffic comes through links from affiliated networks (i.e. Twitter feeds) than specific destination visitors - their good content gets marginalized if not for the eyes of the affiliated community worlds this also supports search engine rankings Advantage - Affiliated communities
The short argument is that affiliated communities help build scale and the built communities help build engagement. Worth further exploration as a future blog post...but in the interim, when two online communities fork in the woods - take both paths.