David Letterman got a going-away present his last night on NBC: Big ratings.Friday’s “Late Night,” with guest Tom Hanks and a surprise appearance by Bruce Springsteen, won roughly double Letterman’s usual audience, according to preliminary Nielsen Media Research figures released Monday. In 29 metered markets representing 50.7% of all TV homes, Letterman racked up a hefty 7.4 rating and 27 share of audience, Nielsen said. By contrast, the Friday before, Letterman drew a 3.6 rating and 18 share. If these results hold nationally, Letterman more than doubled his usual viewership of about 3 million households. National figures will be released Thursday. While Letterman readies his new CBS weeknight show for an Aug. 30 premiere, “Late Night” reruns will be shown on NBC through the summer.