Digest of suggested content based on user activity Twitter acquired Summify that employs users' browsing activity, social trends, and other black magic to build email digests of suggested content that are very effective at drawing traffic back to their site. 

I know in Drupal it should is relatively easy to build digest emails or suggest content blocks based on content or tags that the user follows, commented on or authored. I'm wondering how to take the next step towards calculating a relevancy score of suggested content based on the user's browsing on the site, and/or based on the activity of other active users (trend makers) on my site.

I'm asking for guidance to accomplish this using any or all of the following approaches:


existing modules that uses browsing activity of authenticated user to score relevancy of a given piece of content. Or something like this.
paid services that support Drupal and preferably have a module to interface easily with Drupal. One of Summify's competitors?
could Apache Solr help in any way?
This wiki page on d.o has a review of some relevant contrib modules.
http://groups.drupal.org/node/12347