If their profiles are public, you can look at them, but you may not be able to use the information you see to discriminate. It is extremely difficult to use what you learn on social media in a non-discriminatory way. Maybe you'll see that you think they drink too much or that they belong to a political group you do not agree with; these two things might not even be true and may not be reasons that you can legally reject an applicant. You must not violate fair housing laws. It is best to judge your tenant on legitimate reasons such as credit history or eviction history.