You bring up two interesting points, that I feel have a related discussion. I don’t believe projections have a permanent characteristic, but the use of social media can continue to circulate the projected work permanently.
The audience can definitely only discuss and look at the projections during a limited time before they get to social media. It is at this time that social networks like Instagram become important. If the message and visuals are strong, people will photograph or write about it in hopes of continuing the dialogue.
Regarding discussions, as the example in Montreal pointed out, one of the goals of projection is to create a conversation between people at the locations of the projection. I believe that being in that location creates a more powerful message given the size of projection, social media cannot replicate that. Social media is just a tool to continue the conversation that started at the place of the projection.