Yes, many people have this fatalistic perspective about FB but they shouldn't because Zuckerberg can't buy all the competition. Buying a company requires a willing seller[0]. Consider that Facebook itself rejected offers from Viacom (~$1.5 billion), Yahoo (~$1 billion), Microsoft ($24 billion), etc.[1][2]
The bigger companies such as Microsoft can't force Zuckerberg to sell if he doesn't want to sell. Likewise, Facebook tried to buy Snapchat for $3 billion but Evan Spiegel said "no". Yes, Snapchat is no longer a threat but that doesn't mean another superior competitor can't rise up as a new threat that MZ can't buy.
[0] my previous comment with other examples: https://news.ycombinator.com/item?id=15733501
[1] https://www.businessinsider.com/all-the-companies-that-ever-...
[2] https://www.cnbc.com/video/2016/10/21/steve-ballmer-microsof...