Check your Facebook privacy settings, if you use it. Apparently by default Facebook allows your profile photo to be used by 3rd party advertisers to promote services to your friends.
You can turn this off by logging into Facebook using Internet Explorer (Firefox will not work) and clicking "Privacy Settings" under the "Settings" dropdown in the top right corner, selecting "News Feed & Wall" settings, and clicking on the "Facebook Ads" tab at the top.
Update: According to this source, photos aren't available to 3rd party advertisers after all. Maybe.