Yeah, I don’t doubt that LinkedIn is selling such a high CPM, but the problem for them is that I just don’t spend a lot of time in LinkedIn so the opportunity to lever up that CPM is limited. I’d bet that the average time spent when compared to other social networks is low, which means there is less ad inventory. This is something they should be concerned about because I’m right in their sweet spot for a target market and I rarely spend but more than 10 minutes a month in LinkedIn. How much you want to bet that LinkedIn acquires a social networking company in the next year?

Having said all that a $75 CPM is really impressive.

Since LinkedIn knows the professional histories of its users, and can target sales execs, c-level managers, or certain industries, we can imagine high ad rates for advertisers looking for a business niche. Still, we have a hard time believing this is an average CPM for LInkedIn. But we don’t have a hard time believing that LinkedIn’s average CPM crushes that of Facebook, MySpace and other social networks who routinely struggle to get CPMS above a buck.

[From LinkedIn: We're Selling Ads For $75 CPM - Silicon Alley Insider]