Pandora (NYSE:P) listeners are as engaged as ever, but the jury's still out on its ability to survive the early success of Apple's (NASDAQ:AAPL) iTunes Radio.

The leading music streaming service published its metrics for the month of October, and it shows that Pandora's holding up pretty well in light of the Apple challenge. Listener hours for the month climbed 18% to a record 1.47 billion, and active listeners clocked in 20% better than last year at 70.9 million. But it's even more important to dig deeper than this morning's press release to find how Pandora held up sequentially. After all, this is the first full month of iTunes Radio's availability since that platform launched in mid-September. If Apple was really leaving its mark, we would spot it right away by pitting October against September.

Well, the news is mixed on that front. Pandora did serve up 1.36 billion hours of audio in September, so the 8% sequential uptick is very impressive. Even with October's extra day, that's pretty comforting. But Pandora serviced 72.7 million unique listeners in September. We're at 70.9 million in October. Did Pandora lose 1.8 million users to Apple?

Well, here is where we simply can't satisfy ourselves with the prior month's report. We have to go back a year to make sure that there aren't any seasonal issues at play here. Yes, streaming has its ups and downs as summer travels and holiday breaks kick in. If Pandora experienced a similar decline in unique listeners between September and October of last year everything would be cool. Unfortunately, everything isn't cool. Pandora's unique listeners rose from 58.3 million in September of last year to 59.2 million a month later. There was an even larger sequential boost in users in 2011.

The market appears to be ignoring this potentially problematic trend, sending shares nearly 5% higher at the open. Investors could simply be focusing on the spike in listener hours, or maybe they're relieved that things aren't worse. Apple, after all, bragged recently about serving up more than a billion tunes to 20 million users during the first four to five weeks of availability. A month ago I argued that we would have a final verdict once we were fed the metrics for October, but this is still an incomplete meal. Another increase in listener hours won't matter if unique listeners suffer another sequential dip this month. Pandora has been able to swat away tech titans with dreams of mattering in streaming before, but now it has its biggest challenge to date on its hands.