Marcus Frind is the guy who developed PlentyOfFish.com. He started the site not to turn it into a cash cow, but just to learn about how to develop a website using ASP.NET. PlentyOfFish.com was the first dating website that was free. He then learned about advertisements and now (about 7 years later), he makes over $800,000 / month off of his site.
This may seem easy, but a dating site is fairly complex to build. I don't know if you have any programming experience or not, but if you don't, this kind of site will be (and I hate to be a downer) pretty impossible. Sure, there are CMS engines out there (like hipsy mentioned) to help you build a dating site for free, but the problem with these is that anyone can just download and use it. This is a problem not with the way that it looks (because also like hipsy mentioned, you can theme it), but because of the way it WORKS.
You have to keep in mind that dating websites are not blogs. They are completely interactive web sites that people spend hours, days, months, and years doing CUSTOM programming, development, and architectural design. This involves not only developing the front end design (what a user of your website would see), but also the back end database work as well. So, if you're not already familiar with ASP.NET, PHP, Perl, or Ruby on Rails, etc. and can couple it with SQL Server Express, MySQL, or DBase, etc., then you've got a LONG road ahead of you unless you have the capital to pay someone to build it for you.
When I say "a dating website is not a blog," here's what I mean:
With a blog, it's very simple. You are the only person who writes on it. You are the only person who has an account. You are the only person who manages it. You are the only person who has a profile. etc.
With a dating (social networking site), all of a sudden, there are potentially thousands of people who will not be READING your site, but POSTING their OWN information on YOUR site. This means, you need to handle user accounts, you need to allow them to update their profiles, you need to build a way that they can search for other people, you will need to allow them to upload pictures. etc. All of that stuff is just the beginning of the site! All of that will probably come standard with any dating CMS. The REAL hard part is building the custom functionality with your site.
Hipsy says, "providing a significant advantage for using your site versus plentyoffish." Think about this: If you can use a CMS to have a free dating site. Couldn't someone else have the same CMS? This would mean that you would have two domain names with EXACTLY the same functionality, and EXACTLY the same logic for finding matches. So...what's different between your site and the other person who had the same idea as you? The answer: NOTHING!
I'm not trying to be a downer, I would just hate to see anyone pour out a ton of hours of work into building what they thought would be their own free dating site and then it just turns out to be a dud.
When it comes to building social networking sites, you REALLY have to either BE a programmer, or have money that you can PAY a programmer to develop something CUSTOM for you. If not, then your dating site will be just like everyone else's.