necolas/css3-social-signin-buttons — comment on commit 23cd91ee3c782e18262c89337f2112c5d4bf12d4
File:  line 0
By mathiasbynens:

I think you only ran it through ImageOptim once. I get another 3,7% savings when crunching it again.

(You can crunch and re-crunch the same image up to at least three times using ImageOptim.)