It's hard to recommend the best way to advertise since you haven't disclosed any information about the service or who the target audience is. SEO can NEVER hurt. No matter what it is, there is always a way to get traffic for free through SEO. It's just a matter of perhaps being a bit creative with the keywords you are targeting.
Facebook and Google AdWords are also good options. Paying someone to promote your site for you is never a good idea when first starting out. YOU need to take the time to figure out what works and what doesn't. In my opinion, it's silly to give someone else extra money to set up a Facebook Ads or Google AdWords account and spend your money before even you know what the best strategy is. They'll probably end up just spending your money unwisely.
I'm not trying to say anything negative about SEOs, because many of them make a living at knowing the best techniques and strategies for AdWords, and can establish much more efficient and productive campaigns than people may be able to do themselves. However, I would hold off on doing this until later in the game.
Dani
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