The movement of the bubbles doesn't seem to convey any useful information, it's just added flare. I'd get rid of it. (you might enjoy some classic books by Edward Tufte too)
The placement of the bubbles seems to be completely random. You can use your space more wisely.
You are trying to represent growing trends with this visualization, perhaps some sort of component should go into representing that. Maybe you can represent that in your x and y plane somehow?
You're viz looks blurry on retina screens. You can fix this by setting the width attribute of your canvas element to twice what you want it to be, and adding a css style width at what you want the width to actually be.
Edit: I just read the text describing what velocity actually does, nvm, I'm dumb.
But I agree, a separate one analyzing the pictures could be valuable as well. However, Instagram pictures often don't contain much text apart from a wall of hash tags. So the text might not be super important to analyze. Anyways, picture analysis might be slightly more difficult and more resource intensive than text analysis, but very interesting as well.
Not sure, how it could be done otherwise, but it needs to be thought a bit more about. Maybe do something more to the bubbles, like coloring and filling them. Maybe even just showing a number inside the bubble if there is room, or when highlighted.
Also, instead of refreshing the page, you could consider something like Socket.io between the web page and your node.js server to update the values. There might be other alternatives to Socket.io, but then you won't have to refresh the entire page.
Anyways, keep up the good work :)
 setInterval(grabWords, 60000);