Friendster isn't currently the largest social network. The point is that people associate the biggest product of the category with the category itself. Like web search is synonymous with Google.
Look at Coingecko's list of ERC20 tokens, all of the top 60 tokens have a market cap above $1 billion. You can go ahead and remove Tether if you want, there is still another ~$100B there.
Bitcoin is not so dominant as the maxis would like to believe. It won't take much for Ethereum (as an ecosystem) to catch up, and when it does the flippening will come fast.