We had a constant visits/day above 500 and also at least a question per day. Now we're down to 400 visitors/day and I feel it's almost a week without questions, or something close. Perhaps that question contest is in order? I remember Jon Ericson saying we'd have a round of highlighting the considered best questions that matched the site and focusing on that.
I do appreciate that one could hope for more activity on the site, and even that there might be a certain risk of not worrying about too little activity.
However, there is also a considerable risk it looking too much on some statistics. (There are even calls of some elsewhere to remove them entirely, since they just make users nervous while the actual site evaluation is not really based that much on them. Various site graduated without meeting the number of question threshold.)
Actually, at the moment the activity of the site is pretty good. It is noticeably higher than one or two month ago. September is approaching. Let us see what happens then. If the site does not gain traction when the academic years start (at most places), then we might have some thinking to do how to push the site, possibly including such a contest. Now, I really do not see reason to worry. Call me a pessimist, but the site is a lot more active than I had thought in June/July it would be now.
Furthermore, there are other things that could be done then just waiting for new questions. There is quite a bit of polishing to be done.
Those that have some time to spare, please, look through old posts, check if the tags are good, if some editing is needed here or there, perhaps add an answer where the existing ones are sketchy, and so on.
Let us use the calmer time, now, to polish the site for the new academic year!
Perhaps participating in Community Promotion Ads on related StackExchange sites might help? On Math.SE at least?