We run a community platform called Snapzu (http://snapzu.com) which is similar in nature to HN/Reddit/etc (which entirely depends on user submissions, voting, etc) so we right off the bat had to battle the notorious "chicken and egg" problem. It was really hard attracting users when the entire site depends on other users (of which obviously at first we had very little of) for content.

However, we did several (mostly automated) things to increase our user count to 26,000+ members, which is where we stand right now. The main goal was to make sure that each user is catered to and getting exactly (or close to) what they are looking for.

1. Split up the content into several categories (14 to be exact). This allowed us to focus on specific categories such as Science, Earth, Politics, etc. Each category has its own social media account (on Twitter, Tumblr, Medium, WP, etc.) where we share our highest voted RELEVANT user-submitted content automatically using IFTTT. Example: http://science.snapzu.com

2. We also used the same user-submitted content to build our newsletter and send out the top relevant posts of the week for the "tribes" (communities like sub-reddits) that they are subscribed to. This attempts to get as many people to come back and often, a tactic extremely important when the community is dependent on other users coming back and contributing/participating.

3. We create automatically generated "top list" posts for some of our top categories and share them on all our social channels, newsletters, certain relevant sub-reddits, etc. These again are dependent entirely on user-submitted content, and although I mentioned they are automated, they still require some work (for the intro quote, minor curating) but 95% of the work is automated. Each post takes approx 10-15 minutes to create, instead of several hours, and uses the past week's top submissions as the main content. Example: http://snapzu.com/teamsnapzu/weekly-roundup-earth-and-nature...

4. Obviously there were also several things completely out of our control. Reddit had their massive debacle a couple years ago when Victoria got fired (she hosted their AMAs) which created a massive Reddit "revolt". Over the course of a few weeks in July 2015 we had 40,000+ people come in from there, of which approximately 8000 signed up. But that obviously would not have happened if we were not "somewhat known" at this point. We were basically at the right place at the right time, as one of the few semi-known Reddit alternatives/competitors. Because the "Reddit revolt" was somewhat big news, we also got mentioned in several articles (Daily Dot, Inc, Moz, etc) which brought in a few more thousand users.

We're now using many of the things we learned from the entire 4 year process of growing our own community platform to help bloggers (and/or website/business owners) who are struggling with the same things we were (and still are). Many of the growth problems (and eventual tactics) involved are nearly identical mainly because of the chicken and egg problem I mentioned before. If you are curious it's at http://blogenhancement.com. It contains several tools we use ourselves and basically allows bloggers to start, run, and utilize their own communities to get more audience, engagement, content, and revenue. It also ties in beautifully to our platform that we've spent 4 years building and constantly improving on, so it's a natural win/win for both parties.

Hope this helps. Cheers!