I think many of you might find it useful:
Facebook Photo Sizes
-Facebook Cover photo size: 851 x 315
-Facebook Profile photo size: upload 180 x 180
-Facebook App / Tab thumbnail size: 111 x 74
-Facebook Link preview size: min. 600 x 315
-Facebook Photo post size: min. 504 x 504
-Facebook Group Cover photo size: 801 x 250
-Facebook Event Cover photo size: 784 × 295
-Facebook wall post photo size: 504 px wide by varying height
-Facebook shared link thumbnail size: 484 x 252 pixels
Twitter Photo Sizes
-Twitter Header size: 1500 x 500
-Twitter Profile size: 400 x 400
-Tweeted Image size: min. 440 x 220
Google+
-Cover photo size: 1080 by 608
-Google+ Profile photo size: 250 x 250
LinkedIn Photo Sizes
-LinkedIn profile “background”: try 1500 x 500
-Linked in profile picture: 500 x 500
-LinkedIn Cover photo size: 646 x 220
-LinkedIn logo size: 100 x 60 horizontal or 50 x 50 square
YouTube Photo Sizes
-YouTube Channel Art size: 2560 x 1440
-Channel icon: 800 x 800ΞVideo thumbnail: 1280 x 720