Hi Lisa. I am part of Constant Contact's education team and focus on nonprofit education specifically. I encounter this question quite a bit, out in the field; especially with nonprofits. I find statistics are the most helpful. So here you go...
Your supporters do use email:
Email is a powerful compliment to face-to-face and direct mail communications. Email lets nonprofits communicate more regularly, which is essential to having engaged supporters:
In addition to allowing for more regular communication with supporters, email also helps nonprofits:
With all of these benefits of email, it's no surprise that - E-newsletters are the most important communication tool for nonprofits in 2011, followed by website, direct mail, in-person events, Facebook, and media relations/PR. (2011 Nonprofit Marketing Guide)
Hope this is helpful for your nonprofit! If you have any more nonprofit related questions, I encourage you to join into the new thread just created in the Community just for nonprofits - http://bit.ly/NPCommunity. Ask questions and review the content already there. I also encourage you to check out our nonprofit specific live webinars (they are free) - http://conta.cc/Communitywebinars.