There is a way around this -- find an initial target audience (team gamers for Discord, students of particular colleges for Facebook), start out as a specialty site for those to get a nucleus of active users, and then start building out. But the kind of outreach and engagement (and, honestly marketing savvy) required to make this strategy actually work don't seem to be "Googley".

(It also helps to find target communities not well-served by incumbents -- like, perhaps, the multiple communities who interacted poorly with Facebook's "real names" policy. But instead, Google leadership insisted on copying that policy, and in a particularly heavy-handed way, to the point that Google employees couldn't register accounts with nicknames that they were actually known by to colleagues and family alike; see e.g. https://www.firstpost.com/tech/news-analysis/why-googles-rea... )