It's a play on words, a pun. Tumblr is a popular blogging website where people write blogs for free, but tumble means to fall. Stumble means to trip or almost fall, almost tumble. So, I think the author is referencing popular social networking sites while making it sound like these things aren't necessarily good for us. That's how I understand it. "Stumble" could also be a reference to some social networking site, but I have never heard of it.

The last few verbs relate directly to websites. Facebook (yes, as a verb) = Facebook.com (obviously) :D tumble = Tumblr.com stumble = StumbleUpon.com tweet = Twitter.com