Yes, I noticed this late last night, and then again this morning, and notified NING via email. They are well aware of the issue. A mainpage forum needs the "last reply" link, in fact , I believe it is the most important part of each thread. Noboby wants to scroll through what has already been read.
This is the response I got from Ning Help just now:
Thanks for your message. We're sorry about this bug and understand how important this feature is for you and members on your Ning Network. Our Engineers have isolated root cause and will have this fixed later this evening, between 6:00 and 7:00 pm Pacific Standard Time. We will provide another update on Ning Status Blog (http://blog.ning.com) when this is fixed on your Ning Network.