The case of the Twitter spam I don't understand
It's probably not news to anyone on Twitter that Twitter has spammers (every popular service has spammers, it's a rule of nature). In fact Twitter has several forms of spam, mostly revolving around drawing your attention with @-mentions. Much of what these spammers are up to is pretty immediately obvious and thus uninteresting, which is the state of affairs I'm used to. With pretty much all forms of spam on all services, it's almost always pretty obvious what the spammer is up to and what benefit they hope to get out of their spamming.
But not always. Every so often I run into something that is clearly spammy, where the people involved are up to no good, but I don't understand what exactly they get out of their activities. On Twitter the spam I don't understand is certain sorts of follow-spamming, where accounts follow me without any attempt to message me or otherwise get my attention (some follow spam has relatively obvious purposes, for example to get me to look at the account's profile to see some advertising there). When I run into a situation like this, what it says to me is that I don't fully understand the service I'm using and its environment, and the spammers do. If spammers see some advantage to following my Twitter account without me ever following them back, then they understand Twitter better than I do; there's something about the situation that I'm missing.
(As I've said before, spammers are not stupid in the aggregate. If there are a bunch of spammers doing something, it is because it works; it achieves results that they want.)
The corollary to this is that if you run a service and you see spammers doing something mysterious on it that you don't understand, you probably have a problem. Unless you're absolutely sure that the spammer actions are having no effect at all on your service (ie the only thing they're doing is creating logfile entries in private logs), you should assume that the spammers have spotted something clever that you've missed.
(In my case I'm not sure I care enough about Twitter to go digging into what the follow spammers are up to. Note that Twitter is clearly aware that follow spamming is a potential problem, as I've noticed that I don't always get the email notices about Twitter accounts following mine.)