You can use this URL to mine from different devices without needing to log in to your FreeBitco.in account
Is that URL safe to share publicly? It seems not as it contains authentication hash that may or may not be used to login on your account?
Sure. The mining site reveals your user ID but no other data about your account. No indication of your balance or profile information. And you'll notice that there are no menu options either.
Why would you want to share the mining URL publicly? You planning to convince people to mine for you?
I think if your planning to publicly send your url for people to mine for you, people will not do that because they will earn nothing from it.