Cryptocurrency market capitalization is difficult to determine but can still be a good metric to estimate growth of a cryptocurrency (even if imperfect as provided). While I accept these are probably rough estimates they are still useful data-points to follow in an industry where data is paramount.
When I began using CoinMarketCap.com Bitcoin’s market capitalization was $4 billion. Check it out yourself but as of writing (07/05/2017) Bitcoin is sporting a $42 billion dollar market cap. The site CoinMarketCap.com also gives good insight into other popular cryptocurrencies as they rise and fall in popularity. As you probably know I always recommend diversifying and I also recommend utilizing this site to gain research ideas. Don’t put everything into Bitcoin, explore new possibilities as well!