TV’s rollercoaster year turns another death-defying corner this week as the fall season officially gets under way — and primetime’s erratic perf so far is a reminder that Nielsen ratings, more than ever, are telling just part of the story.

Early results are as scattered as the recent stock market averages: Up one day and down the next, only to rise again.

The CW’s “90210” opened big on Sept. 2, dipped in week two, but then bumped back up, despite tougher competish. Fox’s “Fringe” launched Sept. 9 with a loud thud, but bounced back tremendously in week two.

For an industry so anxious to anoint new hits or quickly bury primetime flops, it’s becoming increasingly difficult to do either.

That may be a good thing: Nets with tricky trigger fingers may have to stand down. But it’s also a reminder that returns have diminished so much that no one can really tell what works anymore.

How the nets might perform this week is anyone’s guess, but no one is expecting viewers to suddenly return after spending the past several months far, far away.

Broadcasters ended last season limping toward the finish line — even “American Idol” was down — after a writers strike-fueled winter of repeats drained them of audiences.

This summer didn’t help things, as viewers mostly stuck with pursuits that didn’t involve the smallscreen.

“This has not been an easy environment to flip the lights back on,” says Fox Entertainment prexy Kevin Reilly. “You can’t spend enough in this environment to galvanize viewers.”

Complicating matters, those marketing messages have been lost this year in a sea of big-time distractions. Audience awareness rates for new TV shows virtually came to a halt during the Summer Olympics. Then came the Democratic and Republican conventions, which actually contained a fair amount of drama and viewer interest — further diverting attention from TV skeds.

Now comes an increasingly bitter presidential campaign — plus, as of last week, an economic system in free fall.

But don’t forget to watch “Kath & Kim,” Thursdays at 8:30 p.m.!

Net execs are once again embracing premiere week in the hopes that they’ll together make enough noise to remind viewers that TV is back. ABC has even been marketing the fall launch — slightly tongue in cheek — as “National Stay at Home Week,” promoting primetime launches as an escape from current headlines.

It’s not gonna be easy: The networks must now combat an unusual malaise coming from viewers, who feel like they’re being sold the same old goods as last year.

The problem: Viewers are being sold the same old goods as last year.

The strike forced the nets to hold back such promising freshman fare as “Pushing Daisies” and “Chuck” until fall. But the networks have had to spend a chunk of time reintroducing viewers to shows they probably assumed had long ago been shelved.

And with fewer frosh shows on the fall skeds, there isn’t quite that sense of discovery and excitement that comes with a new TV season.

It’s gotten to the point that net execs wonder whether they’ve dipped below another threshold, a world in which a 2.5 rating among adults 18-49 is considered good enough. Compare that to a decade ago, when hit shows such as “ER” were still regularly garnering a 17 rating in the demo, and shows needed to at least clear a 5 rating to be considered credible.

Fox put a hefty amount of marketing coin behind “Fringe,” for example, but the show came up dramatically short in week one.

Actually, the J.J. Abrams-Alex Kurtzman-Roberto Orci collaboration’s bow two weeks ago was so lackluster that many Hollywood types were ready to write off the whole TV season.

But a funny thing happened in week two. “Fringe” improved its perf by 63% — the best bounce in 17 years of any show between its first and second episodes.

“It really was a progression in how we got there,” says Fox Entertainment chairman Peter Liguori. “(In week one), we had to bring a 100% new audience to the show. Then in our Sunday night repeat of it, 80% of that audience hadn’t watched the premiere (in its original slot). That culminated in week two with a big number behind the return of ‘House.'”

Along the way, viewers made “Fringe” the No. 1 stream on Hulu and the No. 1 stream on Fox.com. As for illegal means, “Fringe” was among the top 10 most-downloaded TV shows last week on BitTorrent.

With shows now available on so many platforms, execs have no choice but consider non-broadcast factors in their decision-making process as well.

NBC Entertainment co-chair Ben Silverman compares the network run of a series to the feature world’s domestic theatrical window.

“The value is then created around the world in exploitation through digital offerings, via streaming, via home entertainment,” he says.

“30 Rock,” for example, is a ratings dud. But Silverman asks, if the show is popular on Hulu and NBC.com, is high on iTunes’ list of downloaded series, sells plenty of DVDs, sells well internationally and eventually fetches a nice off-net license fee, isn’t it a hit?

That’s one way to look at it. But it’s probably not much consolation to network execs who still live and die by a tenth of a Nielsen Media Research ratings point.