I don't believe there's a Newsfeed Share plugin, but that seems like a good idea. Oxwall certainly won't add it to core, but a third-party developer might make one.

You can make pages visible to all members through /admin/pages and that works fairly nicely (can use HTML to spiffy things up). Oxwall, in the future, will be expanding this I believe and will have CMS features. I find the page feature works nicely as-is right now too, but a CMS will be great.

As for the special capabilities, it's entirely possible to do this. I think it might require a couple of custom modifications, but ultimately shouldn't be difficult with guidance. From the top of my head, I would think the best way would be to find a way to automatically force users to follow the admin account (this will likely be custom, I don't think there's a way to do this without a plugin or editing), and then users are actually able to choose deeper privacy options with the Privacy plugin for their own accounts.

go to the post directly (click post time) and you'll see share bottm

DeFender +1, when you click on the date the newsfeed item was created you'll be taken to the separate page with that newsfeed item where you can share it. 

As to the page created by admin and visible to all members, yet is it possible, check the manual please: https://docs.oxwall.org/user-manual:pages-and-menus

as to this one: is it possible to give special capabilities to the admin so that he can have his posts seen by all the users ? And the other users can only see the content posted by their friends?

what kind of posts are you referring to? blog posts, forum posts? newsfeed posts? 

I don't believe he means Social Sharing, rather a Share button similar to what Facebook has (if a friend posts something cool, you can also share it from their profile onto yours).

But yes, social sharing can be done by clicking on the permalink to the post (date is linked), if you have that plugin installed.

yes I mean social sharing 

That's right Bryan, I didn't mean social sharing.

Thank you all for answering me.