Sometimes what happens, socially with older blogs is that they get picked up by a media writer, think Yahoo columns. They'll credit you and link back but won't even ask you. Maybe someone found it and posted on Reddit or tumblr.

Because you have a picture of Gandhi and the title itself is kind of catchy, I wouldn't be surprised someone else shared it on Reddit or even tumblr where the reposts are SO easy, even if they don't get read people repost for someone else...
If you have Googles Blogger you should be able to trace it down. Maybe conduct a G search based on date with that title... Unless you have analytics it would be hard to trace exactly. Maybe there is a software not even I am aware of.

The question I would ask myself is instead is:
Why did people share it. For that you don't have to trace so back. And the answers you get are way more valuable than you finding out who shared it first.
#Unthink