Reading the letter, what surprised me was how political it was, framing everything as a "left vs right" cultural fight. I think if it was up to me I'd probably fire anybody on either side of that debate who started circulating shit like this. As soon as you're on that level, nothing good is going to come of it and you're just going to make a lot of people angry, which is very bad for the business in a lot of differen…
When you want them to go to company parties, the company gym, eat at the company cafeteria, and in the case of Facebook provide company housing, and generally spend a lot of time in the office, then at some point the company is going to discover that their employees have a diverse set of political beliefs as well as beliefs about gender/race/etc, and they will want to express these beliefs over lunch, and at the gym, and at parties, and in social media. They will also want to express those opinions to their friends, which happen to be colleagues. Now you have the big political debates that used to take place in bars and other recreational areas happening in the workplace. That's the cost of the all encompassing office.
Another ingredient in this toxic mix is that a lot of younger people blur the lines between social and professional, and this has been happening for a long time now. As soon as a critical mass lets their work life bleed into their personal life, this is going to end badly and lead to more oppressive workplace structures, as the company isn't going to suddenly democratize decision making. That's why management needs to be very circumspect about the scope of policies they mandate for their workers.
This tearing down of barriers is happening everywhere -- people do personal activities on their corporate laptops, they use their corporate phones for personal calls, they do personal projects on company time and company projects on personal time. They use their social media accounts to talk about company projects, etc.
The only viable option is for the company to retreat from these spheres and to focus on doing your job. They should not ask for social media accounts in the hiring process, nor should they focus so much on "culture". But that's inconsistent with also making the job a lifestyle and keeping the employees working long hours. If you are going to have people spend 50 and 60 hours working closely with each other, then things like political disagreements turn into HR concerns, and are addressed with the same intelligence and care for worker rights as you would expect from a typical HR department.