Filesonic was once one of the most popular file sharing services on the web. After Megaupload was taken down by the American authorities, it also suffered and as of now it is not accessible at all.
Filesonic tried to avoid legal troubles by putting on restrictions on their service to prevent illegal file sharing. Of course, that did not help the cause as traffic fell down massively in the recent times.
The company has not made any official statement on why their service is completely inaccessible right now.