These are two different sites that both have the word “social” in their descriptions.
Propeller.com is a social news site. It’s similar to Digg and many bookmark-sharing sites. Users can submit news stories that they like or think are important (or that they want to promote), and other users can vote for the stories. Stories with the most votes come up on the Propeller home page for everyone to see. Users who don’t like a story, or don’t think it deserves attention, can “sink” that story.
I submit most of my Associated Content articles to Propeller. I started doing this when it was still netscape.com, but then Netscape (actually AOL) moved the social news to Propeller, and is just using netscape.com as a standard internet portal (like your Yahoo or MSN home page, if you have one).
I wrote about the history of the Netscape site here. I also wrote about submitting my articles to Netscape; my articles get higher exposure in search engine listings and thus more traffic. This still holds true with Propeller; its an easy way to promote my articles, and it works.
Ning.com is a social networking site. Actually, it’s made up of several thousand social networks. Ning allows you to set up your own social network. I belong to a few different networks on Ning; they are specific to certain topics or interests. You can find out more about Ning here.