NEW YORK – Keith Olbermann essentially invented the present-day MSNBC, but he didn’t take it with him when he left.

As Olbermann begins his show on Current TV this week, the MSNBC he left behind has survived; its boss says it has thrived. The prime-time focus on left-of-center political talk show hosts that began with Olbermann’s transformation to on-air activist remains. Rachel Maddow, once Olbermann’s protégé, has taken over as the network’s marquee name.

So far this year, MSNBC’s average weekday prime-time audience is 965,000 viewers, or 10 percent more than last year over the same period, the Nielsen Co. says. Fox, easily the market leader with 2.4 million viewers, is down 12 percent in the same comparison, and third-place CNN is up 10 percent, with 770,000 viewers.

“I was surprised that we did not dip at all,” said Phil Griffin, MSNBC’s chief executive. “I was prepared for a 10 percent dip.”

Dig deeper into the numbers and the picture isn’t quite as clear. Viewership at 8 p.m. following Jan. 24, when Lawrence O’Donnell took over following Olbermann’s abrupt departure, is down 6 percent from last year, Nielsen says. Ed Schultz’s 10 p.m. hour is up 29 percent over last year, but MSNBC in early 2010 aired an Olbermann rerun at that hour, meaning Schultz’s audience of 884,000 is being compared with recycled material a year earlier.

There’s nothing uncertain about Maddow’s ascension. She has 1.05 million viewers, on average, this year, a bump of 100,000 over a year before.

Maddow has been the constant in MSNBC’s prime-time lineup. O’Donnell, who had gotten his own show after delivering strong ratings as a guest host for Olbermann, moved from 10 p.m. Eastern to 8 p.m. after Olbermann left. Schultz had been working earlier in the evening and got his shot in prime time.

Each man has attracted attention in recent months for some overheated commentary. In O’Donnell’s case, he went hard after “Celebrity Apprentice” star Donald Trump during the real estate mogul’s flirtation with a presidential candidacy. O’Donnell called Trump “the most deranged egomaniac” in NBC entertainment history.

Schultz was suspended for a week in May after referring to conservative host Laura Ingraham as a “right-wing slut” while on his radio show. He apologized.

MSNBC is still a destination for liberal viewers, but there’s some disappointment that network personalities aren’t more challenging to the Obama administration and Democratic orthodoxy, said Jeff Cohen, an Ithaca College journalism professor and liberal activist.

“I would argue that it was more independent when Olbermann was there,” Cohen said. “His charm, if you can call it that, is that he’s uncontrollable. He’s not a party-line guy.”

The big test for MSNBC comes this week as Olbermann begins competing head-to-head against the man who was once his substitute host. MSNBC has an advantage in reach because the network is available in more than 95 million of the nation’s nearly 115 million homes with television. Current TV is in about 60 million.

Griffin won’t talk about Olbermann or his Current show.

He’s bullish on MSNBC, though, and predicted that within a couple of years his network would even be able to seriously challenge ratings leader Fox in certain hours.

“MSNBC has established a sensibility, a position, a platform,” he said. “MSNBC stands for something and MSNBC is really the place to go for progressives and people who are looking for smart, thoughtful analysis. We’re growing, and we’re putting real effort behind it.”