From my perspective, sorting out the acquisitions of Yahoo and Google is pretty straight-forward.
Yahoo (list of Yahoo acquisitions) has always been about building a directory. ‘Tags’ are just another way to create a directory. From that perspective, purchasing HotJobs (directory of jobs), Flickr (directory of photos), Upcoming (directory of events), del.icio.us (directory of web pages), even WebJay (directory of music) makes total sense.
Google (list of Google acquisitions) has always been about measuring social gestures. Thinking about the purchase of Urchin & Measure Map (gestures within a site), Dodgeball & Jaiku (gestures within a place), Adscape (gestures within a video game), YouTube (gestures within video) from that perspective also makes total sense.
UPDATE 2 Aug 2007:
In other words, Yahoo is the class roster and Google is the yearbook.
UPDATE 9 Oct 2007:
Word is that Google just acquired Jaiku. Wondering if it will find the same fate as Dodgeball (acquired and forgotten about) or if Google has a plan to integrate the two services.