"And if it doesn't work with webmail, you've lost before you've even begun, so don't do something dumb like requiring a plugin. The trick you want to accomplish is that when one person is using your software, it suddenly provides value to that person and their entire circle of friends, without the friends having had to do anything at all. Then, later, you pull the friends into the fold: if one of them starts using the software, they become their own hub, and get the benefit they have already witnessed from a distance."
Truly insightful.