There’s another debate brewing today, in addition to the one over Facebook Comments and “authenticity” (see curated Link). It’s about the viability for startups of building a user base via search engine optimization or Facebook. Chris Dixon thinks SEO won’t work anymore. Danny Sullivan says it still can, but it shouldn’t be a company’s sole strategy. Sarah Tavel builds on Dixon’s argument, and says Facebook is too crowded to be a startup’s primary channel anymore. Inside Facebook offers an intriguing premise – now that Facebook Credits are a big revenue source for the company, it will be more careful about ensuring the effectiveness of its viral channels for apps vendors. The common theme: free lunches don’t last forever, and you shouldn’t build a marketing plan on a single channel anyway.