Interesting: just last night I received an email from the founders of Askolo announcing the their pivot to a service that does exactly this. I believe the name is "Vapor Text".

Edit: here's the email, for reference:

Hi Max,

Since Askolo soft launched earlier this year, our team has been thrilled with the quality of discussion that's taken place on the site. We've realized however, that we could make a greater impact working on other projects and we've made the difficult decision of moving Askolo into maintenance mode. All existing content will remain available on the site but future updates and new features will be discontinued.

We've recently shifted our focus to a new product - Vapor Text - that we're really excited about. Vapor lets you send private messages to friends that disappear once they're read. You can download it now from the Apple App Store: http://itunes.apple.com/app/vapor-text/id563087964?mt=8.

Thanks for all your support and free feel to reach out to hello@ovenlabs.com if you have any questions.

--Ovenlabs Team