It is legal for a landlord to use something they found on social media to make a decision. Certn will highlight high-risk behaviour, however, if a landlord wishes to use a specific post, tweet or pin, they must ensure that the content of the post could be deemed relevant to the applicants' ability to rent. For example, threatening or violent content on applicant’s twitter profile could pose a risk to other tenants or employees and could violate other tenants right to peaceful enjoyment of their property or workplace. This would be relevant. Applicants’ taste in music, political views, family status, race or religion, are NOT relevant to the applicants' tenancy.
Note: We are not lawyers and cannot give legal advice on this.