Earlier quoted context omitted.
I think Mendeley had more advantages over Zotero than the social aspect (which I never used). Mendeley could automatically incorporate your folder of orphan PDFs into your library, automatically fetching the bibliographic info. It had a nice dedicated desktop client for searching your library, correctly bib info, and adding notes and whatnot to papers, which just isn't as nice on a browser.
You might think a lot of things, but your description of Zotero is completely inaccurate. Zotero isn't a browser-based application, and it can do everything you describe.
There is no mention anywhere of any other way of using it except on the download page.
The first time I went there I didn't click on the download button because I didn't want a browser-based app.