Interesting advertising avenue tdg! For billboards it's all about the message, it has to get across very quickly. So I have a comment on your design.

To put it plainly, except to pedestrians (doesn't look like there will be any there), this isn't going to work. It needs to be simple, to the point, and burn the URL into people's heads. The map background is extraneous noise. The "Blockchain social media technology has arrived." is too, and sounds like buzzword technobabble. The "Your-Username" part of it is not necessary either, and takes up more of the ad than the two URLs for Steemit combined.

I'll copy and paste my comment on the tagline from my old post here

"Post. Vote. X." where X was either Earn., Get Rewarded., or Get Paid. All of these words have connotations of spam or low value on the internet, because of all the "free easy money" shit out there that ends up being anything but.

To make a more effective ad, I suggest making the Steemit.com highly visible, and the biggest or equal to the biggest other element, which should be your tag line. An additional sentence saying something to the effect of "Steemit pays you for your blogs" could hammer in the point in plain English.

Excellent initiative, it's got me considering the costs of purchasing advertising time on a very highly trafficked electronic billboard around here.