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?
Edit:
Following the comments this appears to be a regression of this thread: Why an image of clouds made of dots as an ad?
Edit2:
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.