#1
Go to LinkedIn. Find a bunch of group owners / managers for LinkedIn groups that are large and relevant.

Talk to them.

#2
Next identify important bloggers. Websites with high Alexa rankings, perhaps, and which cover entrepreneurship. Ask them about dropping a mention with a link. Also ask about their rates for publishing an advertorial.

If you don't have an advertorial article / press release at hand, pay me to write one. Or pay someone less talented. ; )

#3
After the advertorials are published, THEN you've given people on Facebook and Twitter something to share. It's much better to share a link to an article OFF your website than ON it because it creates the impression that people are talking about you, that there's some buzz, that your conference is something to look into.

At this point, you can go after social media influencers. Doing so before might be premature, in my opinion.

#0
Forgot to mention this because it seems like a no-brainer. But make sure your conference website has a compelling landing page whose verbal and visual content makes the most of your star speakers and the networking opportunities to be found among attendees.

Photos, writeups, information, and a streamlined sales funnel for booking tickets. That should be ready in advance of everything else, since links are only as useful as what they link to.

Speak to Niche - their acquisition by Twitter probably would net you some decent results in this space. https://blog.twitter.com/2015/welcoming-niche-to-the-twitter-team

I've worked with Pure Matter and was pleased with their performance in expanding our voice and promoting the material and they have a number of affinity relationships that could assist you - let Bryan know I sent you! Good luck.