Many users register, but don’t do any activity

I see many users getting registered on my blog, but they don’t do any activity. A large number seems to be coming from similar looking ‘Username’ or ’email’. Makes me to suspect that they are exploiting some functionality, like either leaving some span, mis-using my database somehow, or mis-using my email module to send spam (though nothing is apparent). Why else would someone (or more people) just keep on creating new usernames without creating any blog entries or leaving any comments? Subscribing to my feed does not need a username, right?
Is some hacker working on my site?