In my web app I have both the user's username, and their Full Name. Username does not allow spaces.
To identify a user, should I show a full name or should I show their usernames? The names would be people who have created a "Friendship" with me.
(This is different from Usernames vs. Real Names, as I do have both data already captured. I want to decide what show show up with profile pictures, messages and such.)
This question would require some more detail to definitively answer. The type of service your web app would provide will dictate this choice. However, I'm providing a general format based on which you can make your decision.
The basic difference between the username and the full name is
A username provides a unique identity on the platform
The full name is a relic of the real, physical world and hence, reinforces connections outside the digital world
Social networking based web apps can be basically divided into the following categories (ref: The 6 Types of Social Media by Out:think):
traditional social networks (Facebook, Linkedin, Google plus etc): Their USP is allowing users to interact with people they know or are acquainted with in real life (or, in the case of Linkedin, know someone who knows them). For these services, being able to view users' full name is not only a plus but a necessity. The username does not matter much in these cases.
microblogging (Twitter, etc.): The value on these platforms is provided by the content users can view and share with people sharing similar interests. In these cases, the two users interacting are often strangers outside the digital world. Providing the username is essential here, often because the users' full name is not relevant to the value of the content being shared. On the other hand, showing the full name would increase the resemblance with traditional social networks and allow users to also interact with the people they know in the real world.
social news, media and link sharing (Reddit, Instagram, YouTube): Like microblogging platforms, the value here is provided by the content and not the connection. So the username is essential here too. Withholding the full name would make the platform conducive for sharing often-times controversial information (e.g. Reddit, YouTube) while providing a full name increases users' trust in the content since it indicates an association with the real world.
So, based on the above information, categorize your web app's service into a specific domain and take your decision accordingly.
Some users don't like to display their real full names even if they've signed up using it.
If you're accepting usernames the user should be given the luxury to use it over their real name. Give the user the authority to choose which one they would like to use.
You can ask a user about his choice from username or real name as his display name.
I would prefer showing the full name.
Usernames can be anything under the sun,eg. 'liferocks', 'rockingkool' 'mrsupersmart' etc. whereas actual names would be far more sensible (or atleast there is a higher probability of people giving a sensible full name as compared to a username). As you said it is to denote who all have become your friends, it would be more sensible if the user sees that John Doe is his friend instead of 'rockingkool'.
If needed you may also show username with less emphasis(parenthize, smaller font, subtle color etc) next the Full name. Eg. John Doe (rockingkool).
If you have a profile picture then estate should not be a problem around the picture to accommodate both of them. Then again, as I said, only if its required.
Edit: Just to add further clarity, my approach with the full name stands for the people in my friend list.
