Facebook has increased the number of display ads in many areas of the site from four to five and sometimes six, which puts about half of the ads below the fold where users must scroll to see them.

The change is confirmed by Facebook’s Help Center, which says that “Up to five Marketplace ads may show at one time on any given page.” On numerous occasions, however, we’ve seen up to six ads displayed on some Pages as of this morning:

Like the last time Facebook made a similar change to display ads, it’s possible that this change will reduce ad click-through rates. Bid prices on these ads likely will not change dramatically, however, for advertisers paying on a CPC basis.  Facebook likely found in testing that adding the extra ad added incremental revenue without meaningfully impacting user experience; recent Facebook 2011 revenue estimates range from $2 to $4 billion.

Additional reporting by Brittany Darwell.