No, in SO's case, their competitors were physically incapable of entering the market (because they couldn't purchase stakes).

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.