Still waiting to see a NoSQL community perspective on this
Since Timeline is more concerned about organizing data neatly than shooting out updates in real time, MySQL is well suited for the app. Although the data is aggregated in the same location as the data is kept (i.e. not over a network connection), that data is managed by MySQL, and not an alternative like NoSQL or Hadoop Hbase.
"A lot of people are surprised that for this shiny new thing for Facebook, we’re using MySQL,” Piantino told Wired. “We treat [MySQL] as a generic engine for data manipulation. We use it as a storage engine. And it’s really efficient."