The more people use Google or gmail, the more data they get for their Machine Learning algorithms and search optimization and the better their searches are. What's the alternative to Google? Bing? It's ok, but even with Microsoft's billion$$, it doesn't have the data to compare searches with Google -- google is just too good. ditto for maps - HERE maps etc. can't compete.
One can build a new search engine based on a different parameter, but it won't have the same level of data (see duckduckgo - it doesn't keep track of you, but the searches are nowhere as good as google
Facebook -- I have friends around the world and they are on that platform. The more people on FB, the more it encourages others to get on FB. I was on Orkut and Myspace too, but the majority of people migrated to FB and FB is the default space for social media along with Whatsapp and Instagram (both FB owned). You could argue about FB selling Instagram and Whatsapp, but these would only dent some of the business and not really take away the money-making section.
Finally Amazon - come on, their profit margin is like 2% overall and they lose money on a lot of stuff. Their business is volume. No one can compete at those prices. Besides which their customer support is terrific - only Zappos is better (and it's owned by Amazon).
For me there is no alternative place to buy e-books, and for buying a lot of other stuff, I find Amazon very competitive. It would be hard to impossible (depending on the product) to move away from Amazon
Apple - yes, it can easily be broken -- unlike the others, it is primarily a product and services company without network effect (except for iMusic and, to some extent Applepay) -- but the apple music area is being overtaken by Spotify and other streaming companies while their Applepay faces stiff competition from a number of other sources
I don’t “Facebook”, but I understand the benefits it may offer people. My teens and their friends view Facebook as something used by “old people”, and that doesn’t bode well for it. I was watching an interview where a talking head described how social media is here to stay but it will change; too many FB users experience negative feelings from it.