Like Facebook? Have all your photos on it? What happens when FB blocks your account? They have almost a billion users and not a single customer service rep you can call.
None of those services are all that unreliable in the traditional sense, the occasional outage is not that big a deal unless the only copy of that document you need is on Google Docs or Hotmail and you can’t get to it.
The biggest problem is that you have no recourse if access to your account is blocked which can happen for any number of reasons.
Make sure anything you care about is in at least three separate places and test your backups. The data you save could be your own.