Advertisers are spending more at niche, or “long tail” web sites, and less at large sites, according to figures from PubMatic, a advertising service for publishers.

The PubMatic AdPrice index shows that, while average ad rates are falling, rates at niche sites are actually on the rise.

eCPMs (effective cost per 1000 ad impressions) for large Web sites (more than 100 million page views per month) dropped by 52 percent from 38 cents in March to 18 cents in April. Medium Web sites (1 million to 100 million page views per month) were nearly flat, with monetization dropping from 34 cents in March to 33 cents in April. Meanwhile, small Web sites improved their monetization, increasing from $1.18 in March to $1.29 in April.

Here are the details:

The purple bars represent the eCPM at small Web sites.

Unfortunately, it’s not clear from PubMatic’s data whether advertisers are shifting their money away from the web, or if advertisers are just spending it more intelligently. There’s been a lot of discussion recently about the poor performance of ads on social networks, so advertisers may be moving their dollars away from these sites.

The PubMatic AdPrice Index is a broad-based measure of ad network pricing information. It is based on anonymous data from over 3,000 publishers who work with PubMatic for ad network and layout optimization services.