> I am not surprised that digital marketing is not that effective. What does surprise me is why its taken this long to figure that out.

It is effective, and cheap at that. I run a small-ish left wing FB site and Twitter account, and regularly do small scale advertising - either to rally people up for a demonstration, or to annoy right-wingers. Proper targetting makes this incredibly easy, and judging by the numbers it works out just fine.