Ask HN: Google hates me, stumble upon loves me?

After paying for about 3000 visits from stumble upon, traffic to one of my sites has increased to about 1300 in the last 30 days from stumbleupon (non paid) and about 10 from organic google searches, over the last 30 days. This is a 10x increase in traffic over the last 3 months. Has anyone else experienced a strange traffic pattern like this? I have had nearly 0 signups from this traffic so I'm concerned what will happen if/ when this traffic drops off, although it's been increasing steadily over the past 30 days. Your thoughts?