What if I came to you and presented you with a milkshake for you to drink and I told you the milkshake was strawberry despite the fact that it was visually "dark chocolate" - meaning it was dark brown and not the beautiful pink concoction with red pieces of strawberry in it. When you taste it, it is indeed Strawberry in flavor but dark chocolate in appearance. How does that make you feel?
I take the same approach with social media icons. Yes, the color is only half the story but it changes one's perspective of the value or experience if the color is something other than what you expect. One could argue that as long as the shape is there (the 'F' for Facebook or the abstracted bird of the Twitter icon) and as long as the link still associates with the intended web site then everything is fine. But the color is equally important. Colors for these icons have become a standard and have become synonymous with the product they are associated with. The same is true for the strawberry milkshake. You expect it not only to taste like strawberry but look like the conventional strawberry milkshake as well.