Not having Facebook or Instagram accounts is a positive hiring signal for me. Having a Fetlife account, regardless of content, is another positive hiring signal for me. (I tend to preferentially hire nonconformists.)
Also, the last point is incorrect: being kinky is not a protected class, and one does not even need to be kinky to be on Fetlife. Not hiring someone because they are on Fetlife isn't protected class discrimination even if their Fetlife profile happened to disclose they are in a protected class. It's a social network like any other.
Interviewing someone opens oneself up to a potential lawsuit, period. Asking for information about a candidate that has nothing to do with membership in a protected class (such as "show me your public websites", which is what such a request amounts to) is no more risk than asking in what city someone went to university (the city for which, for example, could suggest they attended an HBCU). The whole process is fraught with risk; we should not allow that to prevent us from learning a detailed picture about a candidate's lifestyle and character (assiduously avoiding specifically seeking out information associated with protected class membership).
It's already common to ask for a LinkedIn or GitHub profile for hiring certain types of roles. There's no difference asking for Facebook or Instagram or Twitter.
Would you hire a photographer without seeing their Instagram?
It's also entirely legal to discriminate against someone for the specific set of social media services they do or do not have an account on, as long as they aren't serving as a proxy for protected class discrimination (eg ChristianMingle).
All of hiring is a process of discrimination: against the less skilled, less diligent, less suited to a particular job role. More information helps in that process. One has to always be careful to avoid discriminating against protected classes, but most attributes of most people, even outside of the professional sphere, are not protected and are legal and prudent grounds for hiring discrimination.