Create complimentary tools that your target audience want to use. This is aside from your main service, but some services like Airbnb have been able to grow users like crazy by giving users tools that help them do their job.
Case Study:
Airbnb was having trouble getting home owners to post their listings on Airbnb, so they thought of creating a tool that'd help people post to Craigslist when they posted on Airbnbthey were already doing. So with the added feature, it was pretty easy for users to switch over to Airbnb, because if they used the listing tool provided by Airbnb, they could post to 2 different platforms at the same time, saving them time.
