It's not the numbers, it's the drop. If you Alexa didn't see you last year it also didn't see you this year. Whatever the deficiencies of the ALexa system and even if the hard numbers are wrong, the drop may be pretty accurate.
Take a look at other news forums. That is the relevant comparison.
"If you Alexa didn't see you last year it also didn't see you this year."
People have become much more aware of, and concerned about, spyware and adware, and an ever-increasing number of internet users are inadvertantly "zapping" Alexa as a result.