The word "share" gets misused in some chain letter schemes, and this is intentional. People are not likely to question and determine the intent of the viral scheme when it is called "sharing" which sounds so much more appealing than the word "spamming."

The difference?

Sharing:

When a person writes their own content that is not and never was part of any viral scheme.

Spamming:

When a person posts, sends, or tags others in a chain letter such as a blog meme, email random survey or some other thing that tells them to tag/forward/send/share AKA pass it on.