It's got nothing to do with being viral. Viral is when it asks you to enter your email address + password and invite your friends from a list. If something ha

It's got nothing to do with being viral. Viral is when it asks you to enter your email address + password and invite your friends from a list. If something happens to you in a social network, you get an email about it. It's a must for any successful social network of any kind. If you don't have this feature, your social product is most likely going to fail. As a developer, you cannot be afraid to send these types of emails. As long as you give the option to turn off email notifications from your friends or opt out of the service altogether by deleting the account, then all is well. It's not about being viral, it's about user re-engagement. Facebook knows this, and that's why you get an email notification when any little action happens to you. This is what allows them to have 200m+ users logging in EVERY SINGLE DAY. I find it completely acceptable. If you want to launch a social product without this feature, good luck not tanking.