Jackie, you've had us all stumped on this one for a while now, and I'm not sure I have a great answer, but it's an EXCELLENT question.

I think (and this is like 'How do you pronounce often?' - as soon as someone asks me, I can't speak unconsciously and decide what I really do say) that I log 'on' to the Web / Internet and 'in' to the system / network at work.

Assuming this is true, I think the difference is:
- the Web / Internet are open systems without clear boundaries. As they don't have boundaries, I cannot be 'in' or 'out', so I go for 'on' and 'off'.
- the company network has a clear boundary. It is possible to be 'inside' this boundary and logging 'in' is the process whereby I get inside.

Now, this is all a bit hazy, and I suspect I use 'in' and 'on' about the company network, but I NEVER log 'in' to the web.

Not sure if that is more or less confusing than your original question?!?