One argument I could see is that the canonical url is ambiguous where the m.* url is more likely to be intentional. If I am on my mobile phone, I will probably

One argument I could see is that the canonical url is ambiguous where the m.* url is more likely to be intentional. If I am on my mobile phone, I will probably type facebook.com into the address bar, because switching keyboard context for the url would be more difficult. If I am on a desktop I am unlikely to type in m.facebook.com, unless that is the url that I really want. If you think about it this way maybe it would make sense to have a d.* and m.* url. The URL without a suffix will remain the ambiguous case and you will be redirected based on which platform you are on. If you would prefer a specific version of the site, you could request it explicitly. Maybe some type of prompt to easily switch versions if it recognizes a url/platform mismatch would be helpful in the case of shares that you described.