I think the OP just meant that once someone like reddit moves into your cloud, your performance goes down. Meaning, the hardware that my little blog is on works great until that hardware also hosting reddit, and then my blog takes a beating because reddit is thrashing away with actual activity.
Not if the cloud you're running your workload on is large enough. The Amazon Web Services cloud could absorb many Reddit sized sites without any other customers experiencing an impact. AWS is _enormous_.
Not in the sense of running out of machines, but at least from my past experiences if your VMs share physical hardware with someone who is using their maximum IO, yours will suffer as well.
It makes it more challenging to load test. When there is no contention you can usually 'burst' to use more of the machine's resources, but you can't necessarily trust that you will always have that capacity.
