Facebook is now showing 4 ads per page (instead of 3) for a good majority of pageviews. This just went into full effect September 28th.

Cons:
- The 4th ad is partially or sometimes completely below the fold. Sometimes you’ll be served ad impressions and your ad won’t even be seen by the facebook user.
If your ad is only partially seen or completely hidden, that means a lower CTR and higher CPC for you.
- More ads = more competition, which means your ad will be clicked less.

Pros:
- Opens up more advertising space. The more inventory Facebook has, the more smart marketers can advertise in competitive demographics as long as they have good CTRs.

Nifty Fact:
InsideFacebook.com reports Facebook revenue should reach $1.1 billion in 2010.