Facebook's Like button allows the company to mechanical turk content rankings which could challenge PageRank's authority. Facebook could (and probably will) combine this Like data with their own PageRank-like link analysis, adding another layer of relevance, one which they would solely own. Facebook web-search feature coming out soon?
<i>Facebook Optimization may be the new SEO</i><p>A death sentence for the "like buttons", if I ever heard one.<p>(If the 'darkside' SEO turn their attention to it, I'd guess the buttons will be gamed into uselessness in six months flat, with no recourse in sight.)