Thursday night, Letterman got his fourth-largest tune-in ever when he had Oprah Winfrey on — a 10.1 rating and a 24 share, meaning that 24% of all sets in use at the time were tuned to his show. Leno, by contrast, had a 4.6/11, which is roughly what he always gets on a Thursday.
Last night, as predicted here, things went right back to normal. Leno had a 4.5/10. Letterman had a 3.6/8, meaning that he got about a third of his Thursday tune-in. 3.6 is about routine for The Late Show on a Friday.