Truncating a username is a bad idea. Usernames are used to uniquely identify a person on your site, so if you're randomly hiding part of the name, that's defeating the whole purpose of the username.
With such a limited amount of space like the example posted here, it would be more effective to just get rid of the usernames altogether, and only show the pictures (linking to the profile on click, and tooltipping the username wouldn't hurt either).
If there is a little bit more space available, then @JohnGB's suggestion of not wrapping the ellipsis is critical. Maybe even fade out the username into the ellipsis to make it extra clear that something is missing. But this should be a rare occurrence. If usernames are frequently getting truncated, the design is broken.