There probably isn't one best marketing method, and there's no way to know. Try a small run of all of them and see which work.

As far as getting press coverage, you're better off writing a guest post about something valuable and interesting, then tying your site in at the end. For example, you might write about "10 best ways to get hired", then have the last paragraph be about you and your site.

Source: I asked a guy how he got on Venturebeat. http://venturebeat.com/2012/12/05/getting-a-good-deal-on-seo...

Thanks. I think that's a good idea.

I also thought about writing a longer and high quality blog posts which offer a lot of value for job seekers, and then promote the blog post via Facebook Ads.

You ought to try out adwords. You can reach millions of people looking for a job. Test out dozens of catchy phrases. And see what keywords are driving a positive ROI. You might want to also do some display advertising to drive traffic; the clicks are cheaper, but the conversion rates can be lower.