On Facebook you can create a fan page -
http://www.facebook.com/pages/create.php (Create a Page | Facebook). If you try to sign up a company using the regular profile creator, Facebook can usually detect that and will instruct you to create this "fan" page. (The regular profile creator is for actual people.) Creating a fan page allows members of Facebook become "fans" of your pages. You can upload images, information, links, etc and even email your "fans."
I have not created a company page using myspace, but can look into it if you'd like. All I can see right now is the ability to create a page for a comedian, musician or filmmaker....but I will try and dig a little deeper.
Let me know if you have other questions! :o)
