> In theory a spammer could make non-commercial, non-misleading, non-fraudulent speech, and a site like twitter would then actually have to think about questions like first-amendment. I can't say I have ever received or seen spam like that.

While I don't think I have seen it on Twitter (then again I only read it when it's linked) I have seen plenty of it in some older forums & IRC. Generally it's just nonsense like "jqrfefafasok" or ":DDDDDD" being posted lots of times in quick succession, often to either flood out other things, to draw attention to poster or to show annoyance about something (like being banned previously).