WE LOVE THIS QUESTION. Have you used your site for other content purposes like blogging? If you're a directory, you could reach out to reputable companies (or whatever you're a directory for) and ask them to either guest post on your blog and then share it on social media, or you could use link building tactics to get people to link to your site. Make sure you add the value to your directory or aggregator, and make it clear what the purpose of your site is.
One thing you could take advantage of is social media. Since you're an aggregator, you probably have a niche or target, and you can reach that audience to funnel traffic to your website through social media. Other than that, we would just recommend making sure your backend is as tight as possible, and make sure it's optimized for mobile, since mobile is a huge factor. Another thing to look into would be to optimize for voice search.
Hope this helps, and looking forward to seeing other responses!