Communities for start-ups tend to take one of two approaches.
The first approach is the most common. The community manager is hired to manage the growing community. They respond to questions. They distribute information. They bring feedback from the community to the organization. They create and manage Facebook/Twitter accounts.
This is a scalable form of customer service. It's useful, but limited in scope and benefits.
The second approach is less common, but far more beneficial. Here the community manager is hired to connect and grow an audience of passionate believers in the start-ups cause. The community manager works to build a strong sense of community amongst individuals.
This isn't an advocacy campaign designed to foster normative commitment, it's community building process to encourage advocacy through affective commitment. You want members to continue supporting the organization through their belief in the cause and their commitment to each other.
You need a cause before you begin the community. You need to know the meaning your start-up gives to the world. You need a hosted place for those who believe in that meaning to interact with each other. You need to find people whom share that passion. You initiate discussions, establish goals, set tasks, and provide plenty of opportunities for members to contribute to the cause. Giving members influence is important.
You also need regular events and activities to facilitate these activities. Hackathons are great, but so are fundraising nights and any event that encourages a large amount of progress in a short amount of time.
Gradually they'll attract new people, you establish future goals, you let members build their social identities around your organization's efforts.
At the moment we have a lot of start-ups doing customer service, but very few effectively building genuine communities around their efforts. Focus less on the technology and more on the meaning it gives to the world