That's like saying monopolies don't decrease the competitiveness of the market, just the "competitiveness" of their own products for their own consumers.
No it isn't. Monopolies have the ability to crush or buy any competition that arises, which clearly decreases competitiveness of the overall market.
Facebook or Google doesn't have the ability to crush or buy any competition that arises, that provide a more open platform? Which world are you living in?