The humongous scale of users and data e.g. 500 million + users , 3 Billion+ Likes per day, 30 Billion + photoes stored etc that Facebook has been dealing with has been fairly evident and has been publicly known for a while now. I recently came across a concise answer that reveals in good detail the technology and architecture that is powering the same.
Take a look at http://www.quora.com/What-is-Facebooks-architecture
It is interesting to note that Facebook is itself moving away from Cassandra and using HBase more.
A great resource to understand Facebook tech and architecture evolution over time is present here http://www.infoq.com/presentations/Infrastructure-at-Facebook