Hi Heather, these are great tips: thanks. I've been on Twitter for awhile now, and I agree that nonprofits (like companies) should follow everyone who follows them. Why? Because as you stated, it's a way for nonprofits to reach as many people as possible.
I do agree with Scott though regarding the challenge in having conversations/building relationships with so many followers.
One thing that is very helpful as a way to better manage conversations on Twitter is Tweetdeck: a free downloadable application that allows you to filter people by groups and topics. In addition, Tweetdeck alerts you when others are talking to you or referring to you in Twitter. I highly recommend it. Here's the link: http://www.tweetdeck.com
Hope that helps. Thanks again!http://www.twitter.com/katrinah