Bitcoin is now accepted at only three of the top 500 online merchants, down from five from last year, according to a report by Bloomberg.

Retailers might be turning down bitcoin due to scaling challenges and slow and costly transactions; transaction fees also could be discouraging bitcoin users from making small purchases with the currency, the article said.

"There's a problem with the fees being so high — it does price out certain things," Bruce Fenton, founder of Atlantic Financial, said in the report. "There are some micro transaction use cases — a cup of coffee is the big analogy everybody uses — that are being sort of priced out just because bitcoin is going up so much."

As a counterpoint, Jonathan Johnson, a board member of online retailer Overstock.com Inc., claimed in May that the number of bitcoin transactions had tripled on that site since it integrated bitcoin payments in 2014.