I think Facebook's argument was, if you're using it on several devices, then you can continue what you're typing on a different device! (Like how GMail saves your draft every few seconds). But of course they don't mention they might do analysis on what you wrote but never sent...

And some online shops do that too at checkout if you fill in your email but then abandon it, you might get an email that says "Hey, you didn't finish your purchase!". My worst experience was with a phone provider, I just wanted to see how much a mobile package was priced, but I had to add it to a cart. I put in my real number and email and the fuckers added me to their marketing database, sending me SMS ads. Fuck youuu salt.ch!