The new
traffic powerhouses

Search engine marketing typically entails optimizing first and foremost for Google - then Yahoo, Ask, and Microsoft's search engine. But according to Search Engine Land's Danny Sullivan, the other "search engines" that really matter are "social media search engines."

Case in point: Leading blog TechCrunch gets much of its traffic from Google - and Digg, StumbleUpon and Reddit. These "social media engines" are not merely smaller vertical search engines about which search marketers can say "oh, I'm not up on Digg. I might look into it later," Sullivan writes.

Instead, he says, they're traffic powerhouses that are rapidly becoming the equivalent of major search engines - and publishers or search marketers need to think of harnessing them even before traditional players like Yahoo or MSN.

Though, to term them "search engines" might be somewhat misleading - since social media sites like Digg are built for discovery and entertainment, not search. Sullivan calls them "kissing cousins."