The main beneficiaries of the Universal Windows Platform have been Windows 10 Mobile and Xbox users who have seen desktop applications make their way to across the platform divide. However UWP isn’t a one way street and as a result Windows Mobile apps can be released in the desktop too.
Instagram is probably the first high profile app to make its way to the desktop from Mobile, updating its Store offering to include a PC version as well as the existing mobile app. In theory there’s no reason why it couldn’t arrive on Xbox too at some stage in the future.
Facebook has done a particularly good job of supporting Windows 10, whether that up to date releases of its main Facebook app, Messenger, Instagram or WhatsApp.
Not sure whether this is indicative of an agreement with Microsoft or a grab for Windows users abandoned by other social networks. Either way its a positive thing for all Windows users to hold on to.