Update: The issue appears to have been resolved:
Network issues that affected certain Tumblr users have been resolved. Thanks for your patience.— Tumblr (@tumblr) March 30, 2013
Tumblr was experiencing "network issues" that prompted a system-wide outage Friday night.
The company, which recently announced that more than 100 million blogs were using its platform, provided an update on the situation via its Twitter feed:
Tumblr is still experiencing network issues. We’re working very hard to remedy them. Thanks for your patience.— Tumblr (@tumblr) March 30, 2013
Tumblr has experienced a few more outages of late including one last December and another last October.