It appears someone is trying to promote clouds on the network. I've seen this ad on various sites, and as a sidebar and banner style ad.
What is it for? Clicking it results in nothing but selecting a part of the ad, as can be seen in below screenshot. Which makes it clear the clouds are just made up of white circles. Is there some hidden message?
Following the comments this appears to be a regression of this thread: Why an image of clouds made of dots as an ad?
Yesterday I encountered this sort of add out in the wild on the interwebs. So it appears to be something on the advertisers side, not specifically to SO. This happened when I went to read an article from a local newspaper through facebook, which renders a sort of primitive browser within facebook.