Hi Stefano,
as of this moment there are 678,514 user accounts registered on the Snap! site, and a total of 6,277,072 projects, of which roughly 38% are shared or public. Over the course of the current calendar year so far 86,260 unique users have actually worked on their project(s), so we consider them to be "active". The last 2 months we saw a little over 30k unique "active" users each month (the numbers are higher at the beginning of the school year).
I really don't know about downloads from Github, sorry! Does this help anyway?