2ND UPDATE: It was a great opening for Conan O’Brien on TBS last night. The comedian’s eponymous talk show drew big 4.2 million viewers and massive 3.3 million adults 18-49 and 2.5 million in 18-34 to rank as the top-rated latenight show on TV – broadcast or cable – last night. This was an impressive 2/3+ of the audience he attracted for his debut on NBC’s The Tonight Show in May 2009. Conan ranked as the No.1 latenight talk show telecast of all time on basic cable, beating The Daily Show‘s Oct. 29, 2008 edition. It also topped all other latenight telecasts in basic cable history. The show’s median age was also impressive: 30, the lowest among all latenight shows and half of that for the big broadcast latenight franchises.

Conan towered over its direct competition, Comedy Central’s The Daily Show with Jon Stewart (1.3 million; 690,000 in 18-49) and The Colbert Report (1 million; 550,000 in 18-49) and also beat its broadcast counterparts, Leno (3.5 million, 952,000 in 18-49) and Letterman (3.4 million; 1.3 million in 18-49). While Leno and Letterman are not direct competitors as their airtime is shifted by a half-hour, beating soundly The Tonight Show must have been gratifying for Conan following his dismissal from the show 9 months ago. Despite having direct latenight show competition for the first time, The Daily Show was pretty resilient in total viewers, down a modest 13% from its season-to-date Monday viewer average. The hit was bigger in the 18-49 demographic, 21%. Colbert at 11:30 PM followed a similar pattern (down 12% in total viewers, 25% in 18-49 from its season average.) Stewart opened his show by congratulating O’Brien on his return to TV until he “realized” Conan was airing opposite him. (video below the chart.)

The big launch for Conan follows a brilliant campaign that incorporated savvily social media, especially Twitter, Conan’s first outlet following his exit from NBC. “Conan’s audience has been very vocal online, and he clearly made a smooth transition from Twitter to TBS,” said Steve Koonin, president of Turner Entertainment Networks. “Conan delivered an extraordinary audience and stands out as the youngest talk show on television.” (RELATED: Conan’s Monologue & Opening Video) Conan also provided a boost to his lead-out, TBS’ Lopez Tonight, which logged what appear to be the show’s best numbers at least in a long while. Here are fast national data (live + same day) for the main latenight shows last night. Note Letterman’s big lead over Leno in adults 18-49, which comes on the heels of Letterman topping his longtime rival for the first time since Leno retook The Tonight Show:

Late Night Talk Shows: P18-49 (000)

Conan 3285 Late Show w/ Letterman 1336 Tonight Show 952 Lopez Tonight 883 Daily Show 687 Colbert Report 551 Late Night Talk Shows: P2+ (000)

Conan 4155 Tonight Show 3467 Late Show w/ Letterman 3394 Lopez Tonight 1352 Daily Show 1313 Colbert Report 1010 Late Night Talk Shows: P18-34 (000)

Conan 2451 Lopez Tonight 571 Daily Show 455 Colbert Report 367 Late Show w/ Letterman 364 Tonight Show 350 Late Night Talk Shows: Median Age Conan 30 Lopez Tonight 32 Colbert Report 34 Daily Show 38 Late Show w/Letterman 53 Tonight Show 59

UPDATE TUESDAY AM: Very encouraging preliminary ratings results for TBS’ Conan, which drew a 2.9 household rating, edging its broadcast competition of Leno (2.7) and Letterman (2.5). Of course, Conan has only a half-hour overlap with its broadcast counterparts who start a half-hour later and run through 12:35 AM but a ratings win for him on the first night would nevertheless be impressive.

PREVIOUS MONDAY PM: WHAT RATINGS LEVELS WILL MAKE CONAN A SUCCESS? After a brilliant launch campaign that incorporated social media better than I’ve ever seen, Conan O’Brien’s latenight show is launching tonight on TBS. Expectations are obviously high but what would constitute success for his new cable show? The bar in the 11 PM time slot he is taking over was set very low by his predecessor George Lopez who has been trailing all late-night competition with 600,000+ total viewers, 400,000+ of them in the 18-to-49 demographic. Conan is a lock to improve on that. The question is by how much? While the 6 million+ audience he drew for his Tonight Show debut is probably out of reach, attracting half or more of that during tonight’s TBS premiere would be a big success.

Insiders say that, at least in the beginning, he should be able to stay above 2 million viewers on average, which is what he did at the same time last year on NBC. Demo-wise, anything in the range of 1 million viewers and above will be considered a great performance. Also, at least initially, Conan should be able to beat his broadcast rivals Letterman and Leno in the 18-49 demo. As Jon Stewart just proved with his October victory that it’s doable for a cable latenight show to beat the established latenight broadcast completion.

TNT and TBS are arguably the most established cable networks with distribution comparable to that of the broadcast networks. It also has been charging ad rates comparable to broadcast nets. Cable shows like Conan, The Daily Show and The Colbert Report also benefit by the fact that they only run Monday through Thursdays, excluding the low-rated Friday nights as well as all repeats) As for Comedy Central’s The Daily Show and Colbert Report, O’Brien actually has a chance to upstage them, at least initially, because of the timing for his launch. The Comedy Central shows thrive in busy political times. With the November elections behind us, there is enough political fatigue to give Conan an opening.