Do you think all of the 500k were actual Google Reader users? It's possible a portion are jumping on the hivemind's bandwagon. Not discrediting the cause, just saying that some people like to join causes for the sake of joining causes.
But what cause do you join by signing up for Feedly? I understand that even people that did not use Google Reader may be upset that the company drops a service that is based on an open standard, but I can't see why such people would react by subscribing to some other RSS reader.
Maybe a great number of them are people like me who haven't actively used google reader in years but are willing to give rss readers another go after reading about them non stop for the last few days.
