I've recently been working on a Facebook application called Second Friends with the intention of giving people a way of integrating a view of their Second Lives into their Facebook profile.
Thus far, Second Friends is pretty simple. To use it, simply install the application in Facebook, then go in-world to Eduserv Island and touch the Second Friends kiosk. This will give you a secret key. Finally enter your avatar name and the secret key into the Second Friends form in Facebook.
Voila. Your avatar name and image (assuming that you have a profile image in Second Life) will be displayed on your Facebook profile.
Click on the 'My Friends' tab in the application to add a list of all your Facebook friends that are in Second Life (and that have installed the application) to your profile.
That's about it so far.
For future reference, I'm also working on a couple of extra features - neither of which is quite ready yet. Firstly, I want to provide a way of updating your Second Friends 'status' (just like the more general Facebook status but specific to Second Life) from within Second Life. Secondly, I want to allow people to create Second Life notes that can be added to your profile alongside the notes created by your friends. At the moment I'm leaning towards doing this by building on the existing BlogHud facility.