There is value outside of money, mainly in our rights. A public post service protects your rights, like free speech and freedom from search.

It is highly unlikely that any private service will respect your free speech; naturally some post which is unpalatable to corporate PR will be censored. Moreover, your post will almost certainly be searched and data mined for additional profits.

The government is required to protect certain rights you have, the private sector is not. There is value in that which we cannot ascribe a monetary value to.