By addressing these fears you will create a better product for your users.
FOMO is a type of social anxiety that is characterized by a desire to stay continually connected with what others are doing. FOMO exists both in the real world, and in a digital space. How often do you refresh your Facebook/Twitter/Instagram feed or check the email during the day? Probably a few times a day. So when it comes to product design, it's always good ideas to beat the FOMO by sending all important information to the user without any effort from the user side. I'm talking about an effective notification strategy that should be integrated into all products where the user might want to receive timely and valuable updates.
Nobody enjoys receiving spam. And the worst thing that can happen is when your own account is used for sending spam. It usually happens when you sign up for a particular service via the "Connect Twitter/Facebook account," and the service has an option like "Allow us to share promo with your contacts" enabled by default. While good products do not do things like that, many users are still afraid of that behavior. Thus, it's always a good idea to reassure the user that their account won't be used for sending spam right in the context of asking for connection.
Commitment is especially hard when they require payment. Remember the last time you visited a website of service, but this service didn't allow you to try it before buying. There is a reason why so many services offer the "X days trial period. No credit card required." This simple design decision allows users to try the service and decide whether or not they want to continue using it.
People are not comfortable sharing their personal information online; we are especially cautious when it comes to sharing our personal financial data. That's why when a product request for some personal information without letting the user know how the information will be used, users can easily abandon it.
Product cart abandonment is one of the typical problems of many eCommerce websites. Users add products to cart, proceed to checkout, but never complete the purchase. What makes them abandon the process? One of the reasons is hidden costs. Some online services still show the product price without tax while others hide the shipping costs because they are high. By showing the final price and be transparent about shipping charges up-front, you have a better chance that users won't abandon the purchase process.