Features everyone must borrow from Facebook

Uhm, why mention Facebook if you don't list three features that are unique to Facebook or were pioneered by them? Seriously, invites, notifications and comments are hardly new concepts. They're good concepts, but they have nothing to do with Facebook.

If you want to mention something that was pioneered by Facebook and that you should copy, then it's the use of real names instead of usernames or handles. This feature is what allowed people to connect with each other over Facebook when having no other communication channel, i.e. reconnecting with old friends or classmates.

Comments could backfire on you. What if you have paying subscribers who start recommending your competitors or free/cheaper alternatives to other paying customers. How do you deal with that. Deleting the comments risks alienating customers. Removing the comments feature would annoy all the customers who now enjoy it. Doing nothing will mean you're effectively giving free advertising to competitors.