Facebook experimented with putting happier or sadder material into its useds' feeds, and found that had a tiny but measurable effect on their emotions.

I don't see an ethical problem in the experiment as such. It seems to me little different from trying different text on your web site's front page, except that they wrote a scientific paper about it.

However, Facebook's power to influence people does seem dangerous.

Here are many reasons not to be a used of Facebook.