A good product manager regularly speaks to customers and users. However, the current product obviously satisfies current customers. So the product manager should also speak to non-customer, i.e. potential future customers and find out their requirements.
The article Identifying Non-Customers for “Customer” Interviews provides some insight into how to find such non-customers.
Twitter apparently asks user to welcome friends back when they log on after a long time.
This is controversial design to me: On the one hand, it is a nice feature; on the other hand, an obstrusion into privacy. This kind of aggressive behaviour needs to be considered carefully before being put into practice.
Your product is probably very easy to use. You are the best proof: Everything works as designed, each button is where it is supposed to be.
Now please keep in mind that you, as the product manager, know the product like no one else. In order to find out if your product is really as usable as you think, you need to speak to the users or potential users. This post illustrates five ways of receiving great user feedback that will help you improve your software product.