Monetizing social graphs
There has been a lot of buzz about Facebook's effective social graph data and games like those provided by Zynga have helped them. That said, I would still bet on Google having a better chance of making the most money off of social graphs because they get to effectively combine data from at least five sources to build accurate user profiles: statistical NLP analysis of GMail, search terms used by people who are logged in to any Google services, friends and business connections from GMail address books, social connections from Google Buzz (which often includes data from other social graphs like Twitter), and in the near future online multi-player gaming.
There is another issue: infrastructure. While I am willing to roughly equate the capabilities for non-realtime analytics of very large Hadoop clusters and Google's internal (original) MapReduce infrastructure, I would bet that Facebook will have problems with their mixture of highly sharded MySQL, massive use of memcached, and some use of Cassandra for their live systems. At least to me, Goggle's infrastructure is the most interesting aspect of the company. Facebook has awesome infrastructure, but Google's is even more so.