Back when I did freelance web design (when “web design” was a term still used) I’d always tell my musician-type clients to buy a domain name. This was a time when bands relied heavily on mp3.com, Myspace, and Purevolume to get their music online. My advice was based on a long-term survival strategy, since sites that offer streaming services in a social media environment could come and go, but you’ll always have a domain name, and assuming you keep up with renewals it’s near impossible to lose a domain.
You “have” a domain because one buys it; streaming media sites allow users, based on the site’s terms of service, to rent a little slice of their server space. You don’t actually own anything on those sites, but a domain is owned by you, or the band, or organization. You can have your social media properties taken away, though unlikely, especially if you happen to express opinions that a gatekeeper finds distasteful. Search engines can and do play funny with their search results, so you can’t necessarily rely on those to get people to find you when your social stuff burns up.
Even if you don’t have a site, you can still use a domain as an email router. For instance, fart.farm is an available domain. I can buy it and set a email forwarder on email@example.com and have all emails that go to that address go to my Gmail address. Email providers also let you connect your domain email, so that when you reply to emails, it shows the firstname.lastname@example.org. Wouldn’t you love getting an email from email@example.com?
There’s plenty of top level domains available. el cheapo. Here’s an official, very plain text-looking list of all of them from ICANN. Here’s a more eye-friendly, searchable/sortable version.