Ask HN: App.net is closing - how to approach their old customers?

As most of you probably know App.net is closing their current service in the favor of starting a new one - completely different. It was the biggest competitor of the start up I'm working on - www.apptlantis.com . I see this as an opportunity to attract new customers and I am wondering what would be the best way to approach their old clients. In general these are developers who are already interested in this kind of service so it shouldn't be very difficult. Any suggestions would be highly appreciated.