I'd also suggest that even in a social networking space, there's multiple types of data and some of it should be stored in an ACID environment (which probably means SQL). You may not care if you can't save a few thousand (or million) status updates to your data store immediately, but you need to care a lot more about your customer profile data (e.g., account, password, etc.) and changes there should be as ACID as possible. Your advertising or subscriber billing models should probably be ACID-backed as well. In both of these cases, that probably means SQL.