Hello to everyone! First let me say this: Great plugin!
In my country Yahoo Messenger is a very popular IM (social) app - even more popular (if you allow me this ''social'' comparison) than Facebook - and people often send links through Yahoo messenger IM app.
You can find the needed code in this free plugin, for the small icon you can use your current yahoo mail one (found in WP Socializer 2.4.5), and if you need a large icon, you can use this one.
Really hope you can implement this in your next update. Thank you in advance!