SO physically owned all the oil-wells. Physical ownership is much, much different than network effects.
Except that in one sense the network effect is even stronger: oil wells can be divvied up by a trust-busting government (and will still function) but some networks only work reasonably when monolithic. This is an argument for a monopoly administered by the government (like for road infrastructure) or a private non-profit (like wikipedia).
With network effects, the barrier to entry is less for competitors to join the market and there is nothing physically forcing a user to use a monopoly. A great competitor has a realistic chance of entering your market. Plus, a slight shift in public perception and a Monopoly can become a failed business in no time.
Network effects may be a significant impediment to a small business attempting to enter a market, but lets not throw around the 'monopoly' label so easily.
I would guess CL has, in most classified markets, more than 80% market share. The amount of market share necessary for monopoly powers varies between industries (based, presumably, on the barriers to entry), but I don't think I'm being casual with the term "monopoly". Especially considering the power of network effects.
Is it not obvious that CL can only get away with its crappy UI because it has overwhelming market share? The fact that it is immune from normal competitive pressures is the defining feature.
Anyway, at this point, neither of us will convince the other. So... good day to you.