Examples

Example 1

Facebook used to be my favorite social medium. Then my mom signed up.

Moms: inescapable since the beginning of time. Since this sentence is talking about just one means of sharing information, Facebook, medium is the correct word.

Example 2

The disgraced mayoral candidate tried to blame his political downfall on the media, but the voters knew it was really because of the scandalous affair he carried on for close to ten years.

In this all-too-familiar example, media is the correct word because it refers to the mass communication industry that covers TV, the Internet, newspapers, journalism, and so on.

Example 3

My two-year-old nephew is an amazing artist. His favorite mediums are crayon and spaghetti sauce.

Sounds like the speaker has a pint-sized Picasso on his or her hands. When you're talking about art, it's generally acceptable to use either mediums or media as the plural form of medium.