A demand by landlords to require tenants to "friend" them on Facebook is perfectly legal because it does not violate any right based on race, religion or national origin. Consider it one of any number of requirements landlords levy upon prospective tenants. If you don't like the rules, then go live somewhere else.
You could maybe make an argument if this sort of agreement was in the original lease for the company. Then it would be a term of the lease. However, this place wanted to make an amendment to the lease after the tenants were already there. There's no way that's legal. However, if I was one of these tenants I would simply create a fake Facebook profile to "friend" the landlord. Then I'd never log into the account again.
Landlords forcing tenants to be Facebook friends with them is an invasion of privacy and should not be allowed. Being a person's friend on a social media site means being able to view personal information about them. It would be like having an online stalker. Though they are landlords, a tenant's personal life should remain personal unless they choose to disclose information.
This is in no way legal. I am certain this would fall under some of discrimination act. For one, not everybody has a Facebook account. Another thing is that "friending" somebody on Facebook also allows the landlord the ability to snoop on the tenants friends as well. Of course, that may be the point.