“Publicize” is a WordPress.com feature that allows you to send your blog posts to several places at the same time you hit the “Publish” Button.
By default all posts that you create in WordPress.com end up on global tag pages and are sent out to many search engines via Ping-o-Matic!, while sitemaps pings notify several search engines of new updates.
Now, in addition to these built in features, you can also choose to send your posts to several third party services. Currently, WordPress.com Publicize supports Messenger Connect, Facebook, Twitter, and Yahoo!. This very post is also a test of the Publicize feature, so it should be appearing at all those Social Media outlets.
Apparently a mention to activate the Publicize feature is now supposed to show somewhere on the top part of the “Edit” screen, but for some reason it’s not showing in my browser. In any case you can turn on the publicize feature, go to the Sharing Settings page in your Dashboard. Here you can configure any of the Publicize services you like.
To choose what Publicize services you want for a specific post, just click the Publicize Edit link and uncheck those services you want.
You can also customize the message that gets sent by typing it in to the Custom Message box after you click the Publicize Edit link.
If you have more questions, you can read more about how Publicize works here