Weird users signing up, is it spam?

Help! I have lots of people sign up as users. That is great, of course. But I am not entirely sure where they are signing up…they seem to have odd usernames too.

1. I have a contact form – I can identify users signing up here as it will come back to me in a specific format

2. People can sign up to my Newsletter – this is via MailChimp and they have a special format in which they send new sign-ups to me

3. People can become a member of my ‘Mingle Forum’, and these indeed are registered as users on my site. However I have very little people posting in the Forum, and for reading there is no need to sign up. It seems a bit weird hundreds of people are signing up to the forum but not posting…

Any ideas? Any ideas how I can find out. I DO have a campaign running, asking people to sign up to the Newsletter, with a link to MailChimp. I imagine somehow the people I am targetting for that are signing up in the wrong way…but I am not sure where or how…I would be grateful for some help on how to go about this.