POSSE, abbreviation of “Publish (on your) Own Site, Syndicate Elsewhere“, is a content publishing model where you post content on your own domain first, and then push out (or syndicate) copies to 3rd party services with links back to the original content on your site. This way, the original version of your content stays within your control and ownership, and your content can be traced back to you and your domain. Other advantages to following the POSSE model include not having to rely on 3rd party services (social media sites and services hosted by big companies) and that your content will be more easily found through any web search engine.
Your audience might be more comfortable or spend more time using social media services such as Twitter or Facebook to find and read new content rather than going to individual websites or using an RSS feed. Following the POSSE model means that you can syndicate your content from your domain to these spaces so that your friends can use whatever service they want to read your content, whilst allowing you to keep ownership of your work.
You might be thinking “well if my friends want to read my stuff on Facebook, why don’t I just publish it through my Facebook account?”. But what would happen if your friends start using a different social media service in future? What if the social media service that everyone uses now changes its terms and conditions or gets shuts down? What happens to the content that you have already published then? If you publish content on your own site first you will always have an archive of your content available that you can link people to across different 3rd party services.