I haven't seen any recent stats for HN companies, but here are some other stats:

StackOverflow tagged questions: Django 36000+, Flask about 3000 (Flask 675 + SQLAlchemy 1880 + Jinja2 376)

Google Groups: "Django updates" 35864 topics, "Django users" 37333 topics, Flask (no user groups)

PyPi packages: Django 2539, Flask (no classifier for flask but somewhere around 260 search results for 'flask')

IRC: #django usually has 500-600 in channel at all times of day. I can't connect right now for exact numbers for flask (assuming there is a channel) but other frameworks like Pyramid are probably equivalent and less than 100.

Now, except for IRC these numbers don't judge current momentum... but they still give a good indicator on community size. For example, in all these areas plone/zope numbers beat django but clearly momentum is towards Django when comparing the two.