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I am a huge fan of building links with tumblr. On webmaster tools, when the domain linking to me is not a tumblr subdomain but is using the tumblr platform, it says these are unique domains and easily puts my site well into the hundreds of different domains linking to it. While I do agree that linking with the same anchor text may have diminishing return you are still receiving page rank and giving the google robots more opportunities to crawl to your site which ultimately still holds a great amount of value regardless of how google sees their ip. It is a myth that google even considers the same IP to be a negative feature. Google sees each page as a unique site (even within the same domain and same IP). If that site is reputable then it can pass on good value and if it is not then it doesnt. So often when people have many sites on the same IP they are not skilled at making them authoritative and thus assume that more sites from the same IP or more links from teh same IP can't add much value which is really not the case. Hope this helps