Develop a good sense of who these type of users are... you might find that they congregate on Twitter - therefore you should focus on building a good following there.
Maybe your demographic doesn't use social media in which case you have find them in person and find the common places of where online do they go to when 'online' and or what do they subscribe to.

Local listings are also a big help when wanting to leverage google searches in your area, obviously good SEO.

Facebook groups could be a good place just as meetup could be... but depending on your target market.

Regarding traffic, it depends on where your target market its, what resources you have already with your personal accounts, and what you're most likely to implement on a consistent basis with your limited time and resources.

In terms of "sign up to your site" I'm guessing you mean sign-up for your email list. Again this depends on your audience, but it can be something simple, like a video or pdf as long as it provides value and your message touches your target on an emotional level. This may take some testing a tweaking and you won't know until you get enough eyes on your opt-in to see if it's working.

You said your offering a service that is free, but it's also a business. Money has to come into play somewhere. To get people talking, help them with their problem. Send me a message and I can send you a link to a service that runs $1 a day that may be of help.

Hi there,

You can follow the organic or the non organic growth path. But first, I believe that you need to identify your ideal customer and then try to look for ways to reach your audience. What kind of service is this?

Yannis

you know... i recenty developed an insurance product that gained substantial ground within a short while... i can share a simple solution i applied. call me for a few minutes and i might be able to help cheers.

The beginning is always the biggest challenge for me, you have to hussle in the short term to get things going.

You say that the service has huge value, but have you tested this with users to make sure their understanding and perceived value of your value proposition is in line with your own?

I have recently read a few books that you might find useful.

1) Ask - Ryan Levesque
2) Inbound Marketing - Brian Halligan, Dharmesh Shah

You can also find loads of information about inbound marketing online.

Good luck