Hitting http://localhost:3030/v1/facebook will return a random proxy address. It will always ensure that no proxy makes more requests than the rate limit allows.
If necessary, it will wait for the correct amount of time to pass before returning a proxy address. Of course, hits to Google don't affect Facebook, so we keep track of their rate limits separately.
It is also possible to POST http://localhost:3030/v1/facebook/localhost:1234 will blacklist that proxy/site combination for 15 minutes.
It is also possible to POST http://localhost:3030/v1/facebook/ with the JSON body `{ "proxy": "localhost:1234" }`, which will blacklist that proxy/site combination for 15 minutes.
This is normally done if you get blocked by the site and need to stop making requests for a while.