I didn't know facebook was useful. Does techcrunch get paid for this?
Anything that lets you interact with anybody else in any way is going to tempt uselessness. If you want to take an objective look at the whole of technology, I'm sure you could sneer at all the people who play video games on desktops, or take joy rides in cars, or chat for hours on the phone, when they could be using the same technology to be productive.
Facebook's like that. You can waste yourself in a mire of hundreds of friends and pointless updates. You can also do what I do: Maintain a calendar and let friends invite you to their goings-on; write essays and discuss them with a closed network of friends; discuss goings-on in said closed circles. Facebook is a utility. It offers you a lot and lets you decide what you want. Most people choose to waste that tool. They're allowed to. It doesn't lessen the effect of Facebook itself.
The vast majority of apps for the iPhone are purely for leisure as opposed to productivity, and the Facebook app is by far one of the best in terms of usability and completeness of featureset (i.e. not a dumbed-down version of the full web interface).
Useful is the definition of Yelp or Google Maps.
Productive. Well, perhaps a GTD app.