Don't read too much into that - "Twitter spammers" are already keeping an eye on trending searches and posting unrelated spam content with high-ranking keywords. Your data will be skewed by spammers trying to ride the popularity wave.

Twitter has now disabled search altogether which is the first time I recall that happening. I'm sure that's one of the reasons for the decline in the graph.

I'm not sure I understand what you mean. Even now, 8 hours after the news broke, 6% of all tweets are still mentioning Michael Jackson. That's comparable to the peak mentions for the Lakers when they won the NBA title and Facebook during the name rush.

Oh, I was referring to the "Trending Topics" and search box on Twitter itself. They were removed and I suspect that would reduce overall Twitter activity (b/c it's more difficult for people to see what other people are saying).