The one that most others moves to. Until then, being a pioneer in terms of moving early to a new professional networking platform with little people on it just seems pointless.
This is actually a great question because LinkedIn has become ubiquitous in the professional world and I cannot, for the life of me, think of anything that is currently available to replace it. Could someone build it? Sure. Could someone gain the mindshare (and marketshare that it has)? Someday?
Easy to say flippantly, but my colleagues (let alone my network) are distributed across 10+ countries on four continents. My local 'meatspace' has nothing to offer me career-wise. A professional network needs to be online.
But it doesn't need to be through a third-party website. It's entirely possible to cultivate and maintain such a professional network by directly communicating with people. That's what I do, and it causes no trouble.
Meatspace isn’t going to work that well going forward because of wfh making physical proximity irrelevant. I’m contemplating a move to a company located in NYC but I live in Dallas (it’s wfh obviously). I wouldn’t have been able to make that relationship in meatspace as I’d never even heard of the company before let alone shook the hands of someone who worked there.