Nonprofit fundraising is quite a well-populated profession. There are are many organizations, consultants, and professionals providing strategy, operations, and tactical support for causes who are improving their fundraising abilities.
A couple of starting places for "how to's" on donor engagement would be to check out the Association of Fundraising Professionals, Indiana School of Philanthropy, the Chronicle of Philanthropy, and Guidestar. There is a ton of resources available as well as connections with practicing professionals.
Depending on the nature of your platform, it may be helpful to involve an advisor/strategist/partner from the fundraising profession to support your user organizations as you roll out the platform.
You might also consider partnering with a fundraising platform. They also provide resources and technical assistance for fundraising beyond just how to best use their platform. Since online fundraising is intimately tied to social media and communications, you may find some very synergistic relationships that significantly grow both companies.