In order to try and identify the password that gives you a particular hash, the only real way is to try all passwords and hash them to see what you get.
What those websites have done is already tried a huge number of passwords and stored the calculated hashes so when you input your hash they quickly look up their database and provide you with an answer. Have a look at this question on Rainbow Tables.
They do not cope with salted hashes, however (well, you can create a table for each salt, but that dramatically increases the size of the table space) - so the answer you have had back may just mean they have failed to look up that hash.
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should be 16 bytes (each set of four-b64 chars corresponds to 3 bytes; except the last with has only one indicated by the two equals). As a byte is two hex chars, this works out right. – dr jimbob Mar 16 '12 at 20:03