Here's the number of tweets including the phrase "slow news day," over the past three years.
May 18, 2012
We now know the Facebook IPO didn't go so well. So this was probably not a real slow news day, just not as big as Facebook might've hoped.
March 30, 2011
July 5, 2012
March 21, 2012
May 2, 2011
He got over 4,000 retweets. Given all the followup stories about the raid, May 2 wasn't a slow day for news, unless you compare it to the previous night.
So basically, days with a lot of "slow news day" activity on Twitter are either actual slow news days, or days when famous people are making jokes about slow news.