A common misconception is that you need to provide at least 500 words of onsite content to have your page rank with Google. Your rankings are dependent on many factors and signals and is not necessarily determined by the number of words on a page; no matter how well written they are.
It all comes down to creating unique content that is not only interesting, but engages your viewers and drives ongoing conversations in the form of replies or comments. In a recent Google Webmaster Help thread John Muller of Google, clarified this exact point.
"Rest assured, Googlebot doesn’t just count words on a page or in an article, even short articles can be very useful & compelling to users. For example, we also crawl and index tweets, which are at most 140 characters long. That said, if you have users who love your site and engage with it regularly, allowing them to share comments on your articles is also a great way to bring additional information onto the page. Sometimes a short article can trigger a longer discussion — and sometimes users are looking for discussions like that in search. That said, one recommendation that I’d like to add is to make sure that your content is really unique (not just rewritten, autogenerated, etc) and of high-quality."
Google crawls everything from full articles to 140 character tweets. Google recognizes that even short comments or articles can be triggers for engaging conversations. There is no magic number; there are no “tricks” to SEO. Creating unique and valuable content and you visitors and ranking will follow.