Facebook occasionally blocks PopItUp for "violation of community standards" even though that's not the case. As PopItUp continues to grow this will happen less and less, but in the meantime you can help get Facebook's flags cleared by reporting it as a false positive.
To report PopItUp as "not spam", please visit this link. If you see an error message stating that "We can't review this website because the content...", just click the "let us know" link and submit feedback indicating that the link is not spam.
If the PopItUp Facebook group is also down, please report it as shown here:
More information on this process can be found here.