Imgur has switched from five characters to seven for the links generated for images uploaded on their network.
They could not have used six characters because Imgur created a six digit long file name for various sizes of the original file uploaded on their servers.
The company spoke about the change:
Choosing 5 characters from 26 lowercase letters + 26 uppercase letters + 10 numerical digests, leaves us with 916,132,832 possible combinations (625). Upgrading to 7 characters gives us 3,521,614,606,208 (3.52 trillion) possibilities. Not only does this allow our users to continue uploading as many images are they’d like in the future, it creates faster upload speeds because the uploader does not need to search the database for quite as long to find an open filename. Due to the increase in possibilities, it also becomes much harder to randomly find images by typing in random characters, thereby further protecting the privacy of your images.