There are a quite a few ways to do this:
Leverage a user group/mailing list/other asset you already have. For example, if you ran a physical dating service, you could encourage your brick and mortar customers to join the site – even offering to pre-fill their information, and possibly give them a nice discount/comp account
Incentivize your first 5-10,000 users – free lifetime accounts, one on one consultations, the chance to win prizes, etc. Then further incent them by giving them an "extra chance to win" for each additional person they recruit, etc.
Use a white-labelled dating software solution to run your site. These solutions often come with the ability to leverage an existing pool of accounts shared by all the sites using the software. Warning: what you get in numbers is offset by having a large pool of members who aren't actually aware they joined your site – because they didn't!
Pay people to generate real user accounts and commit to participating in the site to some extent.
Pay people to generate fake user accounts, across a variety of demographics, to give the site the appearance of being fully populated. Alternately, pay someone for a pre-existing list of such fake users with all associated profile data/profile pics/etc included in a single tidy ZIP file for your site admin to integrate. For bonus points, once you start getting real people to sign up to the site, you can start removing/hiding the fake accounts incrementally over time, until all your users are "realish".
The last tactic even has a name in the industry – "Seed and Weed".
Some other "non-seed" methods you might try:
The media blitz. Just advertise the hell out of it, really pushing a unique selling point to the jaded media. Sometimes, that initial explosion of awareness can snowball into a healthy/sustainable volume of users. But that's pretty rare.
Give people something useful while they wait for enough potential matches to join up – even a regularly updated blog with insights similar to the OKTrends blog might get people interested enough to join… And when you start turning on the spigot of dating matches, they might stick, because they already appreciate your content. Other ideas – some form of integration with their existing dating life – analytics on their profile(s), email replies, etc… A curated events list to give them dating itinerary ideas. That's just a couple to get you started.
Begging. Never underestimate its power! Just ask people to join, and be honest about the likelihood that anything will come of it. Go door to door if you have to. Get it done!
Create a product so new, different, and amazing that people just can't help but sign up to try it out, and tell their friends about it. Often possible to leverage nascent platforms to achieve this – think of the first dating sites to "get" Facebook, for example. Simply by staking claim to empty territory, they were able to ramp up quickly before the traditional "big sites" could react.