Before you get excited about attracting new customers (stage 1 of the funnel) and creating referral loops (stage 3), you first need to focus on your customer retention (stage 2). Retention is the measure of people who tried the product and liked it enough to return. Acquiring new users won't make much sense if they're going to leave soon after trying your product. So while the phase "keep" is the middle of your customer funnel, it is actually the foundation of your growth journey.
Other terms that fall under the umbrella of customer retention (and are sometimes used synonymously) are user engagement and stickiness. User engagement is how frequently a customer is using a service. And user engagement creates stickiness - when the customer becomes tied down to a product or service and can't easily leave, or in other words, when a product becomes habit-forming. Stickiness answers the question, “How likely is a user to return to the service day after day?”
The more a user engages with a product, the more sticky it becomes for them, and the more likely they are to be retained. In this way, engagement drives stickiness, drives retention, drives growth. Note, though, that a product doesn't necessarily have to be "sticky" to have customer retention.
Sequoia "Engagement Drives Stickiness Drives Retention Drives Growth" (link)
Put simply, if a user finds value in your product, they will return. And retaining a customer is a heck of a lot cheaper than acquiring a new one. In fact, it's five to twenty-five times more expensive to acquire a new customer (HBR Study link). So be sure to do everything you can to keep your existing customers engaged and happy. And how do you figure out how to do so effectively? As is the case in every step of company building, you experiment!
But how do I get those first customers to experiment with?
Attracting those initial customers will be through your network, the customer development that you've done so far, and by reaching out to the people who have provided their contact information in Step 6: Broader Interest. You could do additional paid advertisement to acquire new customers to test engagement, but keep in mind that the goal here isn't to improve marketing but rather to get the customers to stay.
Below are two popular models used to increase customer engagement and build stickier products that the customer returns to again and again. We do want to caveat that habit-forming tools must be used responsibly, especially if you are serving vulnerable customers like children.
The Fogg Behavior Model
Created by Stanford Behavior Designer BJ Fogg, this model (link) asserts that for any behavior to occur, you need three things: motivation, ability, and a prompt.
To leverage this framework, you'll want examine how much each of these factors is built into how your customer engages with your solution. If you're weak in any of these areas, it might be the reason why customers are falling off.
The "Hooked" Model
Building on the Fogg Behavior Model, Nir Eyal added two additional steps that turn behaviors into habits creating a looping effect (video link). He calls it "manufactured-desire."