NEW YORK — Facebook’s value is now bigger than Wal-Mart’s on the stock market.

The world’s biggest online social network knocked the world’s largest retailer off the top 10 list of highest-valued companies in the Standard & Poor’s 500 index Monday.

While the switch is mostly symbolic — nothing specific happened this week to warrant it, and the difference between the two giants is not that big — it signals investors’ insatiable appetite for successful tech stocks. Apple, Microsoft and Google top the list of the highest-valued companies in the U.S., and Facebook looks to be on its way to joining them.

A company’s market value is calculated by multiplying the number of shares of stock it has in circulation by the current price of one share.

Facebook was valued at $238 billion in Tuesday afternoon trading, according to FactSet. Wal-Mart was valued at $234 billion.