Have you tried the tumblr high-light feature?
Soon on Facebook too?
I actually use it and like it a lot. It was one dollar - now it is two dollar per highlight! The tumblr highlight is basically a sticker to your post. Only your followers can see it (and you) and only in the dashboard. The high-light is NOT visible on your tumblr blog itself. I don’t think it changes the edge rank (but I am not sure).
You can select the color of the sticker, an icon and some pre-defined wordings like Help!, Urgent, Big News, Coming Soon, Sale!, New Stuff …
What I do like is that the sticker stays with the post forever (at least I assume. Does somebody know?). So when people reshare - they reshare your post WITH the sticker. This can be very viral once a post does get reshared a lot.
Rule of thumb. Your post might be unrecognized even with a banner. But if you share an interesting post that does get reshared (anyway), a high-light sticker can be very effective. At least this is my impression. There are no analytics for tumblr unfortunately.
Good read about social monetization trends.
Coming high-light on Facebook