Was Facebook's Mood Study Too Creepy?

According to Adrienne LaFrance, senior associate editor at The Atlantic, Facebook manipulating what you see on your newsfeed is nothing new, but users were outraged when a study was published that revealed Facebook manipulated what kinds of words they saw on their feeds - happy or sad - in an attempt to find out if it affected their moods later in the week. Was it legal? Ethical? Did it cross a line?