The Perfectly Targeted Ad

In meat space, I was subjected quite by accident to the perfectly targeted ad, for a Staples' grand opening in walking distance just when I needed printer cartridges. For me, at that one moment, that was effective advertising, on the front of a giveaway newspaper. But what was the chance of others happening by that newspaper with the same need as I? Isn't online more effective, and a better gauge and likely predictor?