I think I would like them better if they weren't so in your face.
(I realize that may be by design, but it does add a lot of clutter to the otherwise very clean design.)
We jump through a lot of hoops to make those share links non-contentious.
We aren't slowing down the page load with obnoxious includes (or raising any tracking concerns by including Facebook, Google, or Twitter resources), nor throwing 30 odd options onto every post (we pull the under performing links, like Linked In).
The whole point of these is to help sites in the network grow, under emphasizing them defeats that purpose.
I've already desaturated the G+ logo red a lot, from the official logo. I know the official red is a bit too loud and distracting. However I can only tone it down so much before it's unrecognizable that it's a G+ logo. Gray scale doesn't work, because we actually want people to notice these icons and use them to grow the network.
Jeff closed a similar question on meta.math.SE, but the issue is not quite identical. I see that, as Jin says, the Google+ icon on StackOverflow is indeed quite toned down. But the icon on math.SE is a much brighter red. Here is a screenshot-cum-proposal:
On the left are the actual Google+ and Facebook favicons. In the middle is what the sharing icons on math.SE currently look like. As you can see, the Google+ icon is pretty close to the loud red colour of the official logo. On the right, I've replaced them with equally faded out versions of the Google+ and Facebook favicons. If the current faded-out Facebook icon is good, then I think fading the Google+ icon should be as well. Edit: I just remembered, there's a word for this: it's tint.
Sorry for bringing up this site-specific issue on meta.SO, but as I said, Jeff has closed the question on meta.math.SE...
P.S. I'd also argue that for all icons to stand out equally well from the background, and not have one be unfairly more distracting than the others like the Google+ icon here, their colours ought to be equally far from the background (white, in this case) as measured in a perceptually uniform colour space. I can explain further if Jin would like.
If you look on English Stack Exchange, the Google plus red is hardly noticeable. But the blue Facebook and Twitter icons really stand out, and are not "harmonious" with the overall color scheme. But we didn't even consider changing Facebook and Twitter button colors. It would be confusing to have different color sharing buttons on each Stack Exchange site, depending on that site's color scheme. Well, I think it would.
Also, the colors for Twitter, Facebook, Google Plus are associated with the product/ services themselves. It would seem peculiar if the Facebook button were orange or purple, or even grey. I think that Google plus should remain reddish colored, for consistency.