The landlord (and tenant) can only do what the contract (and, in my case, English law) allows. If the tenancy agreement allows the landlord to vary the terms of the contract in this manner with no agreement from the tenant then he can, and the tenant would be a bloody idiot for having signed a contract like this. More likely, a tenancy agreement will not allow such changes and if the landlord wanted to change the terms he'd either have to get the agreement of the tenant to a change in the agreement (and the tenant would be at liberty to negotiate consideration for the change - e.g. a rent reduction) or give the tenant notice IAW the existing contract terms and then try to get them (or new tenants) to agree the contract with the Facebook, etc, terms in it.
The important thing is that either party can only do what the in-force contract (legally) allows. I suspect (can't be bothered to check) that the original Spotify terms allow Spotify to change the Ts&Cs at any point and the only rights the user has in this circumstance is either to accept or to stop using the service.