I ended up at https://plus.google.com/101120115153580954446/posts when I clicked the ad (same destination and largely identical content whether I am logged into a Google account or not).
The above-linked page seems to be a Google+ account for “Stack Exchange” to post various things: “interesting” questions from various SE sites (similar the SE Twitter account), but also podcast episode notifications, and some other SE-related news. It seems very similar to the SE page on Facebook.
Joel Spolsky recently tweeted that “You can also follow Stack Exchange on Google+” with a link to a Google+ search for “Stack Exchange” (this was right after he tweeted about the symmetry of SE on Facebook and Facebook on SE: facebook.com/stackexchange, facebook.stackoverflow.com).
The ad itself seems to be just one of the SE-promoting ads that SE injects into its sites (questions from other SE sites, site proposals from Area 51, “Join us on Facebook”, “Join us on G+”, et cetera).