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This was the approach we were considering. However, what gave me pause was looking at other social networks are doing. LinkedIn only has 50k profiles. Meez has multiple index files and looks like they are indexing many of their profiles Facebook and MySpace don't appear to use Sitemaps (although they do have a crawlable member directory) WeeWorld has 500 links The point is there doesn't seem to be any consistency. If we end up creating a human readable directory ala Facebook does that achieve the same thing?

As an SEO best practice, there should never be more than one version of any page URL.  Even with Canonical implementation, it's only a signal, not a directive, and not all search engines even pay attention to that.  While it's perfectly acceptable to offer filtering options for people to discover content, filters should be blocked from indexing at all cost.  Leaving it up to Google or any search engine to have to figure out which is the original content is not a best practice even though many sites do it, and even though Google says it's okay. Trying to get one page found simply by inserting different keywords in a URL is also a crap-shoot since that's the only thing that changes about the page itself.  Like buying a keyword exact match domain only to use it as a redirect to the real domain.