If you're goal is just to copy an existing established site, you're going to either have to generate a lot of content yourself or do a ton of advertising. But, if you're creating something truly new and unique that people will love, then you'll likely have no problem getting a community to come build around it.
E.g. Back in 1997 I built a chess website for playing correspondence chess. There were lots of other sites for playing correspondence chess, but none with actual enforced time controls, and even the weakly/non-enforced time controls were very slow (like 1 move every 7 days). Mine was the first where users could choose their time controls, and the time controls were automatically strictly enforced. The only advertising I ever did was when it initially launched was to post in a couple of USENET groups and forums, and it took off from there. Within a few years sites with deep pockets started popping up with the same idea,and the sheer number of sites that do the same thing today make it impossible for a site with no revenue like mine to get any attention. But even with my almost no advertising, it still got to about 15,000 active users at its peak, and still has several thousand today.
E.g. Back in 1997 I built a chess website for playing correspondence chess. There were lots of other sites for playing correspondence chess, but none with actual enforced time controls, and even the weakly/non-enforced time controls were very slow (like 1 move every 7 days). Mine was the first where users could choose their time controls, and the time controls were automatically strictly enforced. The only advertising I ever did was when it initially launched was to post in a couple of USENET groups and forums, and it took off from there. Within a few years sites with deep pockets started popping up with the same idea,and the sheer number of sites that do the same thing today make it impossible for a site with no revenue like mine to get any attention. But even with my almost no advertising, it still got to about 15,000 active users at its peak, and still has several thousand today.