At least right now, that's not true. It's harder to automatically scrape Facebook than a random decentralised system - mainly because decentralised systems need to have the data sharing built in to work. Sharing the data in a structured way is the central idea behind them.
> We can decide to block these requests if they are used against us.
You're not going to be able to block this access from gov only. If known networks become blocked, new gateways can be obtained for less money than is worth mentioning.
> When policies change, data (and information) can flow to places where these policies have no effect.
Again, if you're interested in anything resembling a social network, you'll need to share your data with a lot of people. Unless you want to keep it all private - you can do the same thing on FB currently, and border control will not have access to it.