Too easy to access, there is a world of difference between the police taking a warrant to Facebook and some jobsworth in the local council browsing through data to look for potential victims to harass.
It might already be possible for facebook employees to access your information anyway, and if it isn't then a similar system could be set up in a govt run agency. There are many advantages to having a democratic administration run such a system rather than a profit-driven one --- businesses will always, rightly, put profit before privacy. If we decide that privacy is more important then we either have to regulate better, nationalise, or choose not to use the service. The third option doesn't seem to be getting widespread uptake; where young people are moving away from facebook it is not primarily because of privacy concerns.
As a junior Facebook employee, it is extremely difficult to get real user data. It's possible, but there's a purposefully large amount of bureaucracy required to get it. For me, it's usually not working doing and I end up generating my own data anyways.
So you trust the govt to do right by you. What about the next govt? What if UKIP got in power?
Honestly I think this information is actually too dangerous for anyone to have - companies or govts. I'd like to see this kind of level of information gathering made illegal.
I don't 'trust' them blindly, but if information gathering is necessary for such a service to exist, then I think it's better if the service is provided by a democratic agency than a private company. Nobody is forced to use it.