It seems that the main three social media platforms - Twitter, Facebook and Google Plus - are using similar layouts which give you the opportunity to display a small avatar (sometimes used for a good quality head and shoulders shot) and a larger landscape display area.
Below are some screengrabs of what my social media profile pages look like at the moment.
Twitter has recently introduced a "Header Photo" and have installation instructions on their support website. The size of your image should be 1200 x 600 pixels and you can resize your image really easily using PicMonkey. As you can also see from my example, darker images will probably work better as your bio text is white and there is currently no way to change this.
Google Plus have always had this facility and there are a number of very clever .gif (moving or interactive) images used on profile headers. I found that when resizing my image for Google Plus that I had to include a lot of white space to the right of image because my avatar was covering the banner picture.
(24.09.12 edited to add : This post on G+ gives dimensions and links to cover image templates)
Facebook introduced the "cover photos" at the same time as their (controversial?) Timeline streaming. This time I shall have to include white space to the top and bottom of my image to allow for the size of the space available. Again, PicMonkey have a collage facility available to help you create the optimum header image and this article gives you the exact measurements that you need to create a really interactive or creative take on the placement of the avatar.
(have a look here for some interesting ways to use the Facebook Cover Photo facility)
Update: Facebook has now also included some new rules regarding the imagery on the header. You are allowed no more than 20% text and you are not allowed to display web addresses within the header. Also, you cannot have calls to action or references to 'like' the page or 'share' content.
As you can see, I have chosen to create an advert, of sorts, for my blogs - Typecast and Geekalicious - and the image placement I have chosen fits in with the new guidelines. People are reminded of my online content as soon as they visit my profile pages. I have used the same image across all platforms as I do with my avatar so that people immediately see my design and remember me. I may attempt to get creative with the placement of the avatar the next time I change images.
How have you used your the space for your header profile?
How do you plan on using it?