How you mean facebook killing them ? And this site was probably sold, nobody would shut down a site like that for no reason.
Propeller was started by and owned by AOL. They closed it down because traffic was declining and it had become a place for spammers. In the final few weeks that Propeller was still operating, the front page was full of spam. Their 2.0 re-design basically killed them because the users revolted. Big companies like AOL don't hold sentimental value in websites. Web properties are closed down when traffic starts dying because the resources required to revitalize a declining site are too much for managers to accept. So, rather than invest in reviving the site, it's more economically feasible to kill it completely.
No-one just closes a successful site - there must be some other reason. FB isn't the b-all or end-all.
they could be...since FB is a 25 billion dollar company, they could buy out any number of smaller social networks to add to their own...
They did not sell it to no one. It was a product of AOL and AOL is a big company enough to care about social bookmarking stuffs. They must have thought it was not good enough to concentrate anymore because of so many other bookmarking sites getting fame and new ones trending. It was totally waste of their resources and efforts so they finally thought of shutting it down. It was not yielding anything good for them other than advertising revenues from it.
Never used it myself so i cannot really comment,but i am guessing it was a quality site for backlinks,i can see a few other sites going down the pan as peoples tastes change if you have a big site i guess you have to change with the times other wise they will become just another statistic.