When viewing my profile in Unified Self Service (USS) I see one community by default but there are several other communities in existence in my USS instance. How do I control which communities are presented by default on my profile?
The profile settings display all communities that the user is a member of. There is no "default" community assigned per user per-se, but one can control what communities appear based on membership into each community.
To control the profile displayed communities, review the membership for each community. Any community you are a member of will appear in your profile.
There is no means available to change the order of communities that are presented in your profile.