Spoiler alert: My answer is at the bottom
--- My high level on Google --- Google hires lots of smart people and puts them in a technical environment to be incredibly productive. Working on a social product (orkut.com) I always felt like a bit of an unloved step child. We weren't considered core and never got much attention during company meetings even though orkut was hitting some amazing page view stats. It's a hard core data company and social was never part of their DNA. This will take them time to do assuming the Google social antibodies don't win.
--- Facebook - the mission --- We, to oversimplify it, are building technology and products to help people share their lives and connect with the people that matter to them the most. Love and belonging are right above safety and security in Maslow's hierarchy and that is what we all come to work to build for. If you don't believe in the mission of the company then the rest of this is going to be meaningless for you.
--- Facebook - the good --- Facebook hires really smart people who want to get stuff done. The entire company is set up around giving each person and team as much autonomy as possible and removing barriers to turing ideas into reality. It sounds like hyperbole but our first company value is "move fast and break things" because we know that most ideas that sound great are, in reality, crap and spending 6 months to design something without testing your core idea is a fool's errand. At Facebook, code wins arguments so we solve debates by building things quickly and throwing them over the fence to get data and then iterate to improve it or learn that it was a bad idea.
As a smaller company (I think we are 5K right now) building something with such a big surface area we have to be lean and scrappy. Where some companies have a team of 20 people working on something, we have 2 people. It sounds scary (and it is, see "the bad" section below) but it pushes people outside their comfort zone and frankly that is the zone we should all strive to always spend some time in because that is where you learn and grow. In my time here I have built a bunch of things I had no qualifications to build but with only 50 engineers I couldn't wait for us to hire the right people. In the process I built, watch fail, rebuilt, and learned from many ambitious projects. I'm proud to admit I've brought down a number of Facebook services in the middle of the night and come in the next day, not to a pink slip, but from my manager telling me how well I fought the fire I caused and then we work through the design to improve it.
This is a company that practices radical transparency. At our weekly Q&A where Zuck stands in front of the company and has to answer any question employees throw at him and if he gives a bullshit answer then the rest of questions will be people calling him out. He sets the standard that the entire company lives by, it's part of the reason why we get things done so blazingly fast despite our tiny size. Another reason we operate so quickly is that we don't build bureaucracy as a response to mistakes and as a result decisions get made quickly. Another benefit of the radical transparency is that each team communicates very openly about their goals, projects, and progress which again makes communication and collaboration really fast.
I could write for hours about hackathons at Facebook but the most significant bit is that it creates a time constrained environment that really brings out the best in people.
--- Facebook - the bad --- Not everyone is meant to be put outside their comfort zone. It is a scary place at times and I've seen some good people just not hack it and leave. This is an incredible place to build amazing stuff but it is also a demanding one. We don't measure by effort, it's about what you can get done and the impact you can have.
We are a well known company that a lot of people write about and the articles about us are of two flavors "Mark Zuckerberg, man of the year" or "Facebook is destroying society". A lot of new grads come here and get really down when they read bad press about us and don't have the context that the press is an industry like any other and they want page views and no one reads a vanilla article. I try my hardest to remind new employees of they realize that the news is not reality, what we are doing is reality.
Working on something that over a billion people use also means that you can't change the font size without upsetting tens of millions of people. If you work at Facebook for more than a month you are going to experience people being outraged over changes we make and often times publicly claiming we have made the change for a nefarious reason. Again it's heartbreaking to hear but unlike the news we can't simply ignore this. We have a responsibility to listen to feedback and try to pull out the good from the people-just-dont-like-change. It's not always easy to do but it's an important part of building a service that so many people care about.
--- My advice --- Pick the place where you are most passionate about the mission and think you will be excited to wake up and go to work at…. hell, pick whatever company/startup/project/organization that makes you want to sleep less just so you can do more. It's past 1am and I'm getting back to my project.