Ideally, you build an audience ahead of time, but establishing some authority and credibility in your field. Publish relevant, valuable content via G+, Facebook, Twitter, a blog, etc., and start building your audience. Then, when you want to publicize your startup, you have people waiting to hear about it, who will - hopefully - become fans and help spread the word further.<p>Also, by putting legitimately valuable content "out there" you are, in effect, doing what people call "content marketing". You want to draw eyeballs to your site/blog/whatever when people search for keywords related to what you are doing.<p>See, among other resources:<p><a href="http://en.wikipedia.org/wiki/Permission_marketing" rel="nofollow">http://en.wikipedia.org/wiki/Permission_marketing</a><p><a href="http://en.wikipedia.org/wiki/Content_marketing" rel="nofollow">http://en.wikipedia.org/wiki/Content_marketing</a><p><a href="http://www.startup-marketing.com/" rel="nofollow">http://www.startup-marketing.com/</a><p><a href="http://hackermonthly.com/startup-marketing.html" rel="nofollow">http://hackermonthly.com/startup-marketing.html</a>