Part of what you said is hyperbolic or plainly not true, another part a false equivalence and, in both cases, it greatly misses the point.

To get two points right out of the way:

1. I completely agree that the mainstream media in the U.S. (and in most parts of the world) is very biased and partisan.

2. I also agree that, as private companies, both Facebook and the mainstream media are in their complete right to proceed with their business as they see fit, within the legality of it anyway.
I don't believe whatever it is that they do is a First Amendment (the rule forbidding the government from suppressing freedom of speech, assembly, etc) matter although it can be a freedom of speech (the natural right) matter.

With that out of the way the main reasoning of this post.

The great promise of the Internet in general and social media in particular is that, maybe for the first time in history, the natural rights of freedom of speech and freedom of assembly come together in this amazing technological manner and, at least in principle, without the need for authorization and without the interference of powerful third parties like the government or the elite.

People can exchange thoughts and ideas, coordinate, and interact instantaneously all across the globe without the need of long travel or intermediaries relaying those messages.

There was even an implicit promise, an unrealized one when, in many of the big public manifestations of the beginning of this decade the so called "Social Media Revolution" helped to magnify the voice of the people in the streets, to help them to coordinate outside the graps of their governments (that usually have full control of both the media and the old means of communication like landlines and mobile phones).

And these new companies (like FB, Twitter, Reddit to name a few) capitalized on that claim too, posting themselves as bastions of freedom of speech, the tools for people to change the world, one hashtag at a time.

Now, the damnedest thing: with each revelation like this one it becomes apparent that "the king is dead, long live the king".

These companies, far from providing the means for people to communicate, to freely exchange thoughts and ideas, they try to shape and mold these ideas just like the very tools the government and the oligarchy uses to control the people.

If it is real that Facebook does that (and there is no reason to doubt) that betrays the people a lot more than the examples you mention. I believe people, after all these centuries since the printing press (now synonymous with journalism) are used to the idea that it is biased, partisan and, in general unreliable.

But when it is their very words and thoughts that are being distorted, the ones from their friends, their neighbors, whose opinions and ideas can be amplified or muted at will depending on whether they conform or not to the gateway controllers agenda, that in my opinion is the ultimate betrayal by those companies.

Assuming this and many other suspicious about their behavior is true Facebook, Twitter, Reddit and the social media in general is betraying the people a lot worse than the mainstream media are.

[1] The Social Media Revolution: Exploring the Impact on Journalism and News Media Organizations [2010]: http://www.studentpulse.com/articles/202/the-social-media-re...

[2] If You Doubt That Social Media Has Changed The World, Take A Look At Ukraine [2014]: http://www.forbes.com/sites/gregsatell/2014/01/18/if-you-dou...

[3] Welcome to the social media revolution [2012]: http://www.bbc.com/news/business-18013662