Ning's new boss announced that free Ning communities are gone. Not a big deal. It's a company, and they're free to do what they want. I'm guessing they'll just piss off their users, and the few people that pony up cash to stay will not be enough to keep the company afloat. Add Ning to the deadpool. That's the risk of using a third-party-hosted service. It can disappear or change, and there's not a thing you can do about it.
I'm finding it extremely difficult to care about the Ning announcement. If you use hosted (especially free) solutions, that's just part of the game. Things change, move, wither and die.
There are lots of apps out there that could easily replicate Ning's functionality - even for free - but they still don't solve the core problem that Ning address, that made it so attractive to its users in the first place.
You don't need a server, or any technical skills at all, to start and manage a Ning community. You only need to be able to fill in a form on a web page and click "Create »" That's all. (well, and a credit card now...)
WordPress/BuddyPress and Drupal and any of a long list of others can provide the functionality of Ning. But, in order to protect yourself from another potential service change/interruption, you really need to provide a server. At some point, you need a Dreamhost account or something similar. You need to copy files to the server. You need to configure a database and tweak things. This is where the people that use Ning in the first place are lost. They can't/won't do this. We can argue until we're blue in the face, saying it's easy, saying it's cheap, saying it's necessary, but the vast majority of people simply don't want to manage the technical layers beneath what they see in the web browser. Ning is betting the company that these people will reach for their credit cards to prevent having to deal with technical stuff.
Sure, there will be other hosted services that will attract some of the people fleeing Ning. But none of them can offer any form of guarantee that they'll stay running, or that they won't change in ways that their users won't like.