In the information age, ideas may be seen as fuel for the digital economy.  But only when they spread.  And we need people to spread our ideas.  The singular Seth Godin has listed 20 reasons people spread ideas.  Here are a few choice selections:

#4...because I have no choice. Every time I use your product, I spread the idea (Hotmail, iPad, a tattoo).

#5...because there's a financial benefit directly to me (Amazon affiliates, mlm).

#9...because both my friend and I will benefit if I share the idea (Groupon).

#12...because your service works better if all my friends use it (email, Facebook).

and #18...because the tribe needs to know about this if we're going to maintain internal order.

Read the full list here.