I think it would be good to have it on the View Airline, like the Activity information, with "Online" or "Offline" in green and red respectively.
I also like the idea of the gates circle style indicator, but I'm not sure where to put it.
The rankings page seems the immediately most obvious spot to put it. It would have the benefit of showing everyone who is online fairly easily, and sorts them by value to allow you to locate an airline of a rough size and in a certain continent for potential deals. The problem with this would be that I imagine it might be a fair load on the server, especially when most people probably don't want to see the information when they look up rankings.
An alternative to the rankings page could be to potentially put a list of online airlines on the home page as text or to create a new page for the information (maybe under the Corporate tab - which only has two pages now).
I think the circles could potentially also be put on the Create Routes page, showing which of your competition are online while you are creating/editing routes. This could be useful, as one may not check to see if their competitors are online before editing routes.
Indeed, circles could be put next to airline names in various other spots - such as on View Aircraft when leasing (both as the owner and leaser), next to messages in the inbox and forum, maybe even next to airlines on the Aircraft Market (though that could be a bit over the top - maybe just when viewing the individual aircraft) or the broker listing on the market home page.
