There's a lot going on in this picture, snapped last night in the Castro Muni station (click it for a larger version), so let's begin our reading with the original author's intent. Those irritating PSP ads (as opined about on Gothamist, discussed by a commenter, and complained about by lots of local bloggers) have finally been put in their place -- a billboard. Rather than being slapped up on peoples' walls in their neighborhoods, Sony's finally figured out that advertisements go, duh, in advertising space.
Next: the "added-value" graffitti. By identifying their billboard as a place for graffitti, Sony probably didn't realize that they were inviting further contribution to their work of art. Such is the nature of scrawling; once one piece gets thrown up, everyone else wants to add to it like a game of telephone purple monkey dishwasher. This particular graffitti appears to say "ghetto B+" -- perhaps an evaluation? "Your ghettology merits a B+" the author is saying, which seems like a fair grade to us.
The sticker, in reference to the upcoming year-long closure of late-night subway service, does a pretty good job of speaking for itself. "Yet another year of subway closures? Enough is enough!" Indeed, Señor Sticker, it is. Let us protest this outrage with huffy anonymous address labels. That'll show 'em.