Well, if I recal, you singled some people out publicly. I'm a believer that nothing good comes from that. You're better off sending private messages to posters. But if you much say something publicly, address everyone, not just a few. For example:
Dear Members,
If you haven't made a donation in the past 12 months, the GID would greatly appreciate your generosity. To put into perspective, a date night at the movies with your significant other cost's $30 and it's over in two hours. The GID provides entertainment for some of us 365 days a year. A minimum donation of $30 doesn't seem like too much to ask. Thank you for being loyal members of the club and thanks in advance for your donations.
You know, something like that. But that's just me.