Now that we have the model built, we can play with the variables to see what effect they have. In the spreadsheet above, go to cell B11, and change the Conversion rate for invites (conv%) to 5%. This will make the Viral Coefficient less than 1. Now look at what that did to your population growth. Instead of continuing to grow, it grows to 20 people, and then stops.
On March 6, 2012, Dollar Shave Club launched their online video campaign. In the first 48 hours of their video debuting on YouTube they had over 12,000 people signing up for the service. The video cost just $4500 to make and as of November 2015 has had more than 21 million views. The video was considered as one of the best viral marketing campaigns of 2012 and won "Best Out-of-Nowhere Video Campaign" at the 2012 AdAge Viral Video Awards.

Instead of buying lots of individual bonds, you can buy a bond ETF to diversify among many bonds and leave the selection to the ETF managers. Bond ETFs come in many different varieties including government, corporate, short-term, long-term, junk, municipal, international and in variations and combinations of each type. Like most investments, higher yields mean higher risk. So choose your bond ETFs based on your risk tolerance, asset type, and liquidity.

Word-of-mouth marketing is a key component to the growth of a small business. It's often word-of-mouth marketing that keeps small businesses running in the early days of operation when there is little to no marketing budget. The consumer shares their experience with your products or services, and they share it with their family and friends. It increases your consumer base and increases your sales.
Viral marketing or viral advertising is a business strategy that uses existing social networks to promote a product. Its name refers to how consumers spread information about a product with other people in their social networks, much in the same way that a virus spreads from one person to another.[1] It can be delivered by word of mouth or enhanced by the network effects of the Internet and mobile networks.[2]