There are a few comments here questioning why it's really important to actually delete data. This is a serious issue for a few different reasons. If I believe that I've deleted something, then there should be no way for anyone to retrieve the information -- I no longer have to worry about security breaches at Facebook (internally or externally), Government warrants, or Facebook accidentally reinstating the information.
As for the idea that Facebook is to big to effectively delete information, that's unacceptable. If you're that big and you can't figure out a way of routinely deleting data then you need to find a way of collecting more data or making your data easier to delete, or not giving the user the 'delete' option. There are a number of alternate verbs which describe the process they are going through, none of which are as clear or commonly understood as 'delete', but which are more accurate. 'Hide' and 'make data invisible' come to mind.
I don't have as much of a problem with the saving of messages which others will also read, but on deleting a Facebook account, this could be handled more gracefully if the user wishes. This is a more difficult problem to solve, which would require Facebook itself to store messages with under the covers public key cryptography which sounds like the type of thing they wouldn't do.