Facebook is, arguably, the biggest social media website worldwide -- When you have as many people visiting and using the site as they have, it should be mandatory to have every point of view represented. You're fragmenting a big part of your user-base by picking and choosing what major pages are being removed.
All opinions and points of view should be easily expressed on Facebook. I mean, we had to suffer through all those crazy gaming app requests, remember? Now we suffer through strategically targeted ads and search results. Facebook rather touts itself as the social media platform for everyone. Athiests belong in that group if I am not mistaken.
Social networks are exactly that: social. They're places for people to chat, get to know each other, and exchange ideas. However, Facebook seems to disagree: they took down pages that promote atheist ideas. If they're a "social" network, I think that was a bad move: their network should be a reflection of society and should therefore allow room for everyone's perspective.
Facebook is a private company and does not have to ensure free speech to all of its users. If Facebook finds that certain points of view hurt their business then they can remove that content from their servers. The company must be profitable and keep shareholders happy. If many become offended by controversial content posted on Facebook, it may hurt the company's bottom line; making investors unhappy.