It's quite active! Three people working full time on development plus more people for marketing and operations.
Check their Github: https://github.com/NebulousLabs/Sia/commits/master
Also, their devs are very friendly. If you join Slack you will find them there as well chatting with the community and answering questions.
In my opinion, the coin is much undervalued for what the tech has achieved already. For sure a good time to invest.