New answers tagged tumblr
Did you possibly ask them for support? The Whois info for that IP is http://whois.arin.net/rest/nets;q=18.104.22.168?showDetails=true&showARIN=false&ext=netref2.
I couldn't find anything that indicated that Tumblr has a crawler. And typically "good" crawlers usually identify themselves as such. So this is probably just someone at Tumblr either viewing or reviewing your blog.
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