I suspect the submission of spam correlates to your traffic, more than anything else. I have some sites with a "contact me" form which get hammered, and others

I suspect the submission of spam correlates to your traffic, more than anything else. I have some sites with a "contact me" form which get hammered, and others that get one or two hits a month. (Adding simple captchas, or hidden fields, etc, can work at the low end. The ones that get hammered though have bots/people that deal with that.)