Mb is unambiguously megabits but it's a lost battle at this point. This thread includes so far MiB, MB, Mb, mb all to mean megabyte.

It's often easy to find what's meant from context but sometimes we're discussing network stuff and it becomes hard to figure out what people mean by "gb" or "mb" or other frankenunits because the context refers both to data in transit and at rest. It's almost certainly not gram bit and milli bit which helps narrow it down...