For that matter, Google knows probably as much as any entity can ever hope to know about people's browsing behaviors. They track what sites people click from their search results, they'll know if you go back immediately to search, or try a different search, if you're not happy with the results. They have Google Analytics installed on millions of domains and know intimately the behavior of users on those sites. I really fail to see how the simplicity of a binary "like" button on opt-in pages with easily-spammable RDF tags can compete with this.