It's definitely a UI issue, in that right now the burden of management of multiple identities falls to users, and yes, developers don't usually include facilities to help. It's a pain, and like Wolfrom I sometimes choose to not say anything at all.

But beyond that, I think it's also a problem of expectations mismatch: on an app level social media is about siloing information by person (or possibly by company), but the user expectation is often that they've siloed information by subject matter.

It's a burden most acutely felt by so-called renaissance men and women (which I'd wager are an over-represented group here in the HN community). There's discrimination towards people that talk about/do seemingly unrelated things, that I think Wolfrom is alluding to. People tend to confuse specialization with expertise, e.g. that a software developer that also does visual design and has an interest in making music, for example, isn't as much of an "expert" as someone focused exclusively on code. Unfortunately not separating your content can have an impact on your credibility.