The "surface" of Facebook may not be, but the parts of it that keep "personal information" certainly are, due to the scope of what can happen if it leaks.
Edit: people take my comment to mean it won't be a big deal. It will be. However, not on the same scale of taking out the power grid, or the water system, which would lead to hundreds or thousands of deaths. Facebook is not critical infrastructure.
If it leaks, there is a direct impact on users monetary expenses.
One of the examples:
FB may know their user's lifestyle - eating habits, drinking habits etc. (based on the content/media users upload) If this info is leaked to insurance companies, it'll have direct impact on the premium you pay.
Edit: people take my comment to mean it won't be a big deal. It will be. However, not on the same scale of taking out the power grid, or the water system, which would lead to hundreds or thousands of deaths. Facebook is not critical infrastructure.