I was hanging out with a friend last night and she said, "I think Facebook is designed to distract you. You go in looking for one thing, but oh, hey, there's this other thing, and what's Aunt Marsha up to? Eventually you find yourself spending hours on it."
What she's describing is the Gruen Transfer, "the moment someone experiences after entering a shopping mall when, surrounded by an intentionally confusing layout, lose track of their original intentions." You enter saying "I came to buy a coat" and leave "Ooh, shoes! Cameras! Necklaces! Smoothies!" that much poorer. Modern mall architects deliberately go for this effect; their objective is to keep you inside the mall as long as possible, in order to increase retailer's opportunities to sell to you.
Facebook appears to have hit a Gruen Transfer state completely by A/B testing, which if you think about it, is exactly what A/B testing of ad-driven sites is intended to do: keep your eyeballs in front of the advertiser's windows for as long as possible. Facebook has just had more time, more money, and more talent to throw at the issue than anyone else. The objective remains the same: to distract you from your objectives, to encourage an increase in sales. Your pleasures and interests don't enter into it.
This may be why Twitter is "experimenting" with their newsfeed. A/B testing shows them that they could keep your eyeballs on their interests for much longer; the only question remains how corrupt they want to deviate from their original mission.