From what I remember (can't test right now), a zero-width space is okay as long as there are other (printable) characters in there too. This seems reasonable, because allowing a tweet to be a single zero-width space would make it appear be empty and probably lead to some confusing display issues.

I'm pretty sure I've used it to "end" a hashtag early, like in this made-up example:

    I've eaten two #banana<ZWS>s today!

In my language, the possessive form doesn't take an apostrophe ("Alices Adventures" instead of "Alice's"), so for hashtags and user names it can be desirable to use the ZWS as an invisible apostrophe.