I remember awhile back watching a video where either David Heinemeier or Kevin Rose started about how crucial it is to start with live data for a new web app. Where's the best place to purchase up to date local company data?
If you're talking about a dating site, then stealing profiles from around is not legal. However many dating sites run partner programs which allows you to basically develop your own GUI for their database.
But seriously, it is my opinion that you can get new members to register on an empty site if your project is unique and has something very good to offer. It's something every newstarter should be doing - innovating.
If it's identical to the rest of the competition, then your chances include either a massive marketing campaign (expensive) or partnering with an existing market player.