If you want to share screenshots via Facebook, there is an easy way to do so with Greenshot: simply use the personalized upload e-mail address Facebook has set up for you.
In case you do not know your upload e-mail address, you can find it out very easy, Facebook’s Help Center entry tells us how to do it:
Open the mobile upload page located at https://m.facebook.com/upload.php (you do not have to use a mobile device to do so). If you are not already logged into Facebook, you are prompted to do so now. At the bottom, you will see your personal upload email address.
Once you know it, the rest is quite easy: Take a screenshot with Greenshot and use the E-Mail feature with your upload email address to upload it to Facebook. If you want to add a description, you can do so by typing it into the E-Mail’s subject. That’s it :)
P.S.: Of course we are aware that Facebook offers interfaces to upload images directly from a desktop application. But as there is no Facebook Plugin for Greenshot yet, uploading it via email is currently the easiest solution to share your screenshots on Facebook.