In my piece on Nielsen’s shift to time-on-site metrics, I mentioned the downsides of using a single metric.

ZDNet has a follow on to the Nielsen story that explores this further.

The shift in methodology puts AOL at the top of the list and Google at #5. This can be attributed to two factors:

1. AOL gets a lot of its traffic from mail and instant messaging. From a time spent perspective, these are hugely engaging applications. They are also hard to monetize.
2. The best search engines get the user to what they are looking for as fast as possible. This hurts the time spent number, but provides great opportunities to deliver relevant advertising.

From an investing standpoint, getting people to spend time on your site only matters if you can get advertisers to pay for that time. Otherwise, you’re incurring extra costs to serve those people for longer.

Put another way, would you rather make $300 for 1 hour worth of work or $300 for eight hours worth of work?