Hi Howard,
As for the 'have to', I'd say no, you don't really have to use unique descriptions with your content on those networks. But I'd keep in mind that your audience is on several of those networks because they want to find different kinds of content or want to engage in a different way. Thus by copying descriptions and make something potentially unique generic, you might miss a good opportunity to connect with your audience on one or more of those networks.
Just my 2 cents, hope this helps you