Any traffic that goes from app to web will not have any Referrer, because there was no previous web page before the current one opened, so the browser won't set its document.referrer string.
Weibo and WeChat are normally used as mobile apps rather than websites. So you'll encounter this case.
Having said that, you mentioned that your Referrer is getting set as "linkedin.com"? If so, then could it be that you are using LinkedIn's shortened URLs with your Weibo/WeChat posts? If yes, then the user journey is like this:
Weibo / WeChat (app) --> click linkedin.com shortened link (browser) --> your web page (browser).
Since the linkedin.com shortened link is opened in the user's browser, the browser is able to record that as the document.referrer when it opens your web page.