I browsed back to their site tonight and its' still there and apparently business is growing despite the Yonkley site itself having very little traffic. They seem to have morphed the business from straight development to hosting private label Twitter clones.
The hosted sites allow advertising, mini-cms, editable profile pages and various other features.
I find this fascinating. I can't think of terribly many businesses that would benefit from a service like this, but iSweat.com is one. It's people doing regular exercise programs supporting each other to keep up with their programmes. I'm sure that there are lots of other special interests that could use a service like this.
It's also interesting to me because they have a product for an obscure niche and yet they can find enough customers to stay in business.