Fallon inherited Conan's numbers and had a lead at first with the novelty of a new show and big-name guests.
Maybe people are seeing it for the trainwreck it is:
"Jimmy Fallon may have got a big leap out of the starting gate during his first two weeks as Conan O'Brien's successor as host of NBC's Late Night , but he was edged out by CBS's Craig Ferguson on Monday, according to Nielsen figures. Ferguson's Late Late Show scored a 1.8 rating and a 6 share to Fallon's 1.6/6 -- not a big difference and probably statistically insignificant, but the closeness of the race likely produced some worry among NBC programmers, given O'Brien's consistent winning streak in the time period."
http://www.contactmusic.com/news.nsf/article/ferguson%20pulls%20ahead%20of%20fallon_1098210