Canonical has decided to shutdown the Ubuntu One service, the main reason being the fierce competition between cloud storage services. Many of them are providing free storage in 100s of GBs and Canonical doesn’t have any interest in becoming a leader in cloud storage as it would warrant much more investment in something totally different than their flagship convergence. Both the storage and online music services would be snapped with effect from Jun 1, 2014. User data would be available till Jul 31, 2014 and deleted afterwards. If you have any data on Ubuntu One, ensure that you download the data.
There are plenty of alternative cloud storage options like Copy, Dropbox etc. which have native clients for Linux.
The official Canonical blog post with more details.