Based on a quick search, it appears the dedicated Memorial invitation distribution campaign started 9 years ago:
http://www.jehovahs-witness.com/topic/130927/help-locating-some-info
Given the effort, time, and expense involved in this global annual month-long campaign, one would expect to see -- starting in 2007-- a marked jump in the incremental year-over-year increase.
(Memorial Attendance chart courtesy of JWFacts.com)
Call me stupid, but I don't see any evidence that the invitation campaigns have done a single thing to increase attendance.