Key points about social media (and how this applies to the dissemination of propaganda) from peer reviewed research:

• Information is shared rapidly on social media, most of it within just a few hours of it first appearing online
• Information that is shared on social media continues to live for a long time, generating subsequent click thrus for days and weeks in to the future.
• Information shared on social media is long lived – that is, generates click throughs back to the web site for days (and even weeks)
• “Social media count for 30% of the overall visits to web sites” – more than comes from search engines.
• “At a first approximation, a web search service provides pull based information filtered by algorithms and social media provide a push based information filtered by humans”
• News is the source of a primary topic of conversation on social media
• To reach a maximum audience, content producers should either promote focused content that is the most popular with their (niche) audience or produce a large quantity of content that reaches multiple interest areas.
• “Secondary URLs” (that is not the original or primary source which posted on online but those that are shared on social media by others) generate the majority of web traffic.
• 59% of shared links are never clicked – which means only the “headline” is read by other social media users
• Shared links on social media “creates a majority of the visits and hence advertising revenue”.

Source:
Maksym Gabielkov, Arthi Ramachandran, Augustin Chaintreau, Arnaud Legout. Social Clicks: What and Who Gets Read on Twitter?. ACM SIGMETRICS / IFIP Performance 2016, Jun 2016, Antibes Juan-les-Pins, France. 2016. <hal-01281190>. Retrieved from https://hal.inria.fr/hal-01281190/document on January 2, 2017.
I have highlighted some of the findings of this study in regards to their impact on social media propaganda efforts:

• Most items are shared within hours of their initial appearance.
• Most users of social media only read the headline and never click through to the detailed story or report.
• Items posted on social media basically live forever, even when completely false.  This way falsehoods turn into permanent “facts”.

Most influence comes from the initial “headline” (even when false, as illustrated by the Washington Post’s false report about Russian hackers infiltrating the U.S. electric grid). Few people click through and read the actual story, and fewer still will see subsequent corrections.  Thus, the headline is the message.
Once posted online, fake news basically lives forever and continues to influence others for a long time to come.
This is why social media is ground zero for the dissemination of for profit fake news services (targeting left and right with emotionally laden messages and imagery), traditional news services wanting to generate eyeballs for advertisers and for profit propaganda services (such as Occupy Democrats and other online-based, social media for profit publishers).