This is a problem I faced for many years online. Here are the best ways that I personally know of, to attract referrals to any program or website:
1). Friends and acquaintances.
This option is limited, because there's only so many people you can "know" and "communicate with", but these are the people who will be most easy to convince to join anything you're involved with. I am not talking about friends and family. I mean, if you have a web development website and ask all your friends who don't have a website to join -- that would be rather pointless.
What I mean, is that during your activities online, join forums and help people. Get involved in the chit chat. Make friends. Add some to your messengers. If they ever need any help, be there for them. Then, when you find something useful online and want them to join, you can simply go to any of them and say "Hey check this out..". I find, that more often than not, if they're interested in what I have shown them, they will join.
I have over 100 friends on various messengers right now, that I could pop up and say: "Hey check this out".. and they'd go and check it out. 80% of them would probably join too. But then again, I have been online for 10 years, so I have a lot of friends and acquaintances online.
Also, if you're on a forum and someone asks you a question, answer the question publically and help them. Then, go and PM then, and elaborate on the help that you gave to them publically. Recommend a program at the end. 50% of the time, they will go and join. Even if you don't personally know the person, or have them on any messengers. They will appreciate the fact that you helped them, and give your recommendation great consideration.
It's all about networking and making friends and not just taking from people, but giving too. If you go around the world and give to people, many of them will give back.
Why do you think I am offering free hosting with no catch?
2).
I am an avid fan of SEO. I like to create small to medium websites and full them with keyword rich information on any particular subject. Then, I promote that website in the best way I can. I add it to my related websites, add it to high PR directories, and anything else I can do to get it started. I might create a YouTube video, tell the friends I have who might be interested in such a website to go take a look. I go to StumbleUpon and quickly click "I like this website" and sometimes I'll even drop $25.00 and advertise on StumbleUpon.
Then, before to long, one way or another, the website is attracting traffic for those keywords, and I can place ads on those websites funneling the visitors off to different programs, services and resources (usually my own websites).
3). In my opinion, if you're going to get anywhere online, you must have a home base. I have
http://www.mrlen.com PR:0

which is my home base. It acts as a FAQ center and Information Center for zeriouz -- and a couple other websites. This forum is relatively new, because up until recently I had taken a rest from my own websites -- but now I am back in action, and this forum is already growing pretty fast.
You need a place to keep all your friends and website members. I have a link to these forums on all of my websites. I use this forum as a contact center. If anyone has a question, they know where to find me. It is absolutely critical to have a generic home base.
I could probably keep typing for hours, but the above are the most important things. I also use 7search and bidevrtiser and a whole heap of other programs to generate traffic. I am a member of three top, paying safelists -- but the traffic I get from them isn't all that fantastic. I mainly remain a member because, well -- they do send some traffic. It's "just" worth the money I pay them. Some days are better than others with them.
There's also a lot of scams online. I have lost thousands of dollars to scam artists in the past. So My advice is that if you ever hand over any money 1). Try to use PayPal and NOT a credit card, and 2). Always check on Google to find if people are complaining. But keep in mind that sometimes the website or service that people are complaining about don't deserve it. Sometimes poor reviews are made by competitors, which is really low. Or sometimes the poor reviews are made by someone who has a grudge about one thing, so they complain about 50 other things, falsely. You need to use your own discernment and just me careful.