Totally agree with Andy's answer. But I just went to your site and I think you should really optimize your sign-up flow before spending any money or effort to acquire customers. Your current multi-step process and required information is far too cumbersome to efficiently acquire sign-ups. I would rework your sign-up process significantly before doing as Andy recommends.

Andy's advice is reasonable because you really want to test your channels and validate your core offering on a relatively small scale before looking to handle a lot of customers. LinkedIn is actually very effective for you as an advertising channel and though more expensive than other CPC's, it's far more targeted for your initial customer base.

Happy to talk you through what I mean about your sign-up flow in a quick call.

I read about 2 things: drive large traffic and expand to bidding platform.
What you should be doing is the million dollar question: how can I spend $10 to get a sale and profit $20 from it.
Notice: sale, not traffic or user.
Like you mention, there are tons of ways to get traffic but you don't want 15 year olds from India by the million visiting your site. What for, right?
The way I would approach it is via trial and error. What works for your competitor may not even work for you.
But please, never go on all fronts. That never works. Every startup with limited resources that I see create a Facebook Page, build a blog, doing SEO, email marketing and so on, they fail.
I can take a call with you to understand better what resources you have and give you my opinion on picking one channel to try for X amount of time, what to do, what to expect and how to track success.

There is no magic bullet here. But some of the new targetable ad channels are pretty affordable. Facebook, LinkedIn. I would test $100 and see what results you get. You also want to use content marketing and tie it to SEO. Optimize for "How do I find a good accountant" and provide top 5 tips on what to look for in an accountant. Good content will also draw traffic on Twitter and Facebook. Contests are also a good way to draw people in.