Unless a person has an unusual name it is not so easy to find someone on facebook they have no connection to. Usually you would need to tell a person your email address.
If the receptionist invites a guest to be their "friend" on facebook, I would presume that they both had some kind of conversation about it. If not, it would be kind of creepy.
From a business owner's perspective, I think it is better that staff give out the hotel's Facebook page URL, rather their own personal profile. Facebook pages are a good way for hostels to promote their business on FB - see the internet marketing section for more tips.