Let us place that number in perspective. And best of all, this is usually free marketing.
Most sites do nothing about it.
The former is a marketing crime, the latter is not enough. Internet users are busy and impatient. But they do definitely want to recommend your site. You should somewhat force them to recommend your site, but make it very easy to do so and pleasant. And you should remind them at least once, in their face, on every visit to your site.