Okay, so maybe not exactly its prime. But I noticed that in their announcement, Google noted Reader's years of declining usage as the reason for ending it. Now, I have no sense of its popularity; I don't even use it myself. So out of curiosity I went to Google Trends to check interest in Reader:
Now, Google Trends is a pretty rough indicator of the actual number of users of something. But I found it interesting that there's absolutely no sign of a decline until just last year. In fact, the last couple years seem to be the strongest in Google Reader's history.
So either Google Trends is way off, or lack of interest isn't the main reason Google is shutting it down. Perhaps it's a slightly more conniving move to shore up Google Plus. Ars has the details, but Hitler might have put it even better: