To complement the answers already posted, here are some SEDE queries. (SEDE queries have the advantage that they can be run by any user - and they also can be easily run on other sites. On the other hand, site analytics contain also some information which isn't available SEDE.)
I should include the usual caveat that SEDE is only updated once a week. You can also notice that it is possible to change the parameters with start and end date in the queries below. (I have chosen the startpoint in November - so that it is easier to compare the graphs with the ones that have already been posted.)
Here are some examples of queries which could (very roughly) correspond to some measure of activity on a Stack Exchange site.
- Number of posts/questions/answers per week - including the deleted posts
- Number of posts/questions/answers per week
- Number of upvotes/downvotes per week between two dates
Number of posts
Specifically, if we look at the first query - with the number of posts - we get the following graph.
We can also run a similar query counting posts in the whole network. This is what it looks like at the moment:
We can also check how the similar stats evolves on some other sites - for example, StackOverflow Mathematics, Physics, Academia or Math Educators. Of course, there are sites where you'd expect increase of posts, such as Politics or Skeptics and perhaps Law.