I think your posting in the wrong domain if you need help on which domain to choose. Also, while donations work every now and then, you can actually advertise on your site once it brings in traffic via google adwords, etc etc... and try to break even that $20/mo.
Honestly, when you purchase a domain, it all depends on how much you are willing to dedicate your time to in order for the site to become popular.
Looie's making a great point actually. If you're starting this out thinking that you'll surely get your $20 in donations once you have 10000000 users, don't even start this. A $20 hosting package will give you about $20 worth of service.
22. Unlimited Accounts: Unlimited Accounts are indeed unlimited. These accounts are not to be used to host a collection of MP3's, Warez, image hosting or gaming sites, giving away free hosting is cause for immediate termination, or to Backup Your Home Computer.
If you are still trying to come up with a domain idea I would recommend a brandable word to use as your site name. Something pronounceable, easy to remember and catchy. I like the idea you have here, but as ldcd said be careful and read the TOS. Good luck!