According to Real Trends;

Half of all brokerage firm leaders oppose permitting the display of listings on certain social media sites while the other half doesn't think it is a problem. Should such a policy be approved, however, almost 9 out of 10 (88%) agree that the name of the brokerage firm supplying listings to any social media sites should be clearly identified for compliance purposes. And nearly 9 out of 10 (85%) favored an “opt-out” provision for display on non-participant sites.

This means that 50% of real estate companies are against listings being shown on certain social media web sites. When the listings are shown 88% of real estate companies believe the supplying company's name should be clearly identified.

It's simple really. In pre-internet era, any real estate company who listed a property for sale had exclusive right to advertise the property anyway he or she chose. The listing agent/broker could put an ad in a newspaper, magazine or other periodical. When the property was advertised, the property must clearly show the listing real estate company. Advertising another agent's listing was strictly prohibited by rules in most cases.

In the digital era, while the mode of transportation of information has changed to such physical items as laptops, tablets and smartphones, the principle of  showing a listing hasn't changed at all. When a property for sale is displayed, the listing company and listing agent want their information displayed.