This probably sounds really cliched by now, but this seems like the perfect use for influencer marketing. Social networks are always driven by network effects,

This probably sounds really cliched by now, but this seems like the perfect use for influencer marketing. Social networks are always driven by network effects, and having big names there early is what often drives the initial engagement. So approach a bunch of popular people on Twitter/YouTube/Instagram/whatever (including celebrities), and try to incentivise them to use your social network, as well as to advertise it to their followers.