AOL has purchased the blogging search and relevancy engine Sphere. Chief executive Tony Conrad has confirmed the deal to us and lays out some of the details in an official post on Sphere Blog. Conrad declined to state the terms of the deal, but TechCrunch has a source putting the value in the $25 million range.

Sphere first surfaced in 2005 as a blog search engine (our coverage), but by 2006 had evolved into a top provider of contextual content from blogs and other sites (our coverage). The service has partnerships with sites such as The Wall Street Journal, Time, CNN and Reuters among others. Not surprisingly, Sphere was also featured on various pages of its new parent, AOL.

VentureBeat was one of the first sites to use the Sphere plugin, ‘Sphere It’, for WordPress. Some sites use the service a bit differently. The Wall Street Journal, for example, allows Sphere to place links to related content directly on its pages.

Sphere notes that the decision to join AOL coincides with AOL “reinventing themselves.” While this may seem to be a rather generous assessment given that some of that reinvention is little more than pumping out more niche sites to pull in the advertising dollars, comScore’s newest data shows AOL’s Platform A was the top ad network in March — so perhaps the move is not such a bad idea. A larger reinvention will certainly occur if Yahoo and AOL are able to pull of a deal to their rumored merger (our coverage).

The San Francisco-based Sphere had raised $3.5 million in funding over two rounds.