Here's an article that goes through how to add the 'Forward to a Friend' (this shares the entire email) function... http://community.marketo.com/MarketoArticle?id=kA050000000L8IBCA0
You could also add social sharing buttons.
No. I am looking for a way to share an individual article within an email through social media. Many email solution platforms like Constant Contact, Exact Target, Hub Spot offer this.
There is no default social share functionality in Marketo for individual areas. What you can do is add static share links to the sections of your email. I'm guessing you are adding snippets of an article in your email like a newsletter?
Here's an example for Facebook:
If you put that in your email as a link, it will bring up the FB share dialog with the website. Add the URL of your article (instead of marketo.com) and it will create a link to share the article on FB. Some searching for "static share links" for Twitter, Pinterest, etc. should turn up the code for the rest.
EDIT: Here is a list of how to create these in email: http://daniemon.com/blog/static-social-media-share-buttons/