Except that I highly doubt facebook and twitter delete all copies of the data.
Edit the comment once or twice then delete it? They may keep full history though, who knows.
Storage is nearly free today; so much like the NSA I'm sure facebook captures and stores everything. Plus it is text; if they had a space problem they could just compress it and store it to huge tapes if disks were too expensive to do that (probably not an issue for FB).
Do you think Snapchat does?
Even deleted iMessages are marked as "deleted" in the SQLite db but are still around for months.