Your default email is
- Your log-in
- Shown on your profile (What is visible on my anonymous profile?)
- Where you will receive email notifications about pokes and other things
If you want to change it, click your name / profile picture in the upper right corner, then Settings.
Your email will be at the top of the page, so just write in another email adress and click Save.
Click Mail settings below this section to adjust what types of emails you get.