Zoosk Messenger is an application that was used part of a dating website. It's currently available for various mobile operating systems while the Windows one appears to be discontinued. As far as I know for chat applications, you can see who's online only when those users are added into your friends list.
Alternatively, the dating websites might offer the possibility to see who's currently online based on preferences. You should install the application for your mobile device and register for an account to see what are its features and what does it offer.
