If only there were a market index for shares of social media companies.

The combined values of the top privately held social media companies jumped 377 percent in the past year, compared to a 15.6 percent rise in the S&P 500 index, according to a new report.

The report from NYPPEX, a secondary market advisory firm, shows how frothy valuations are getting for Internet companies like Facebook, Zynga and Groupon.

In the most recent quarter, the valuations of these upstarts soared as institutional investors poured on billions of dollars while accredited individual investors snapped up shares on private securities exchanges and secondary markets.

The value of the social-gaming company Zynga has risen 80 percent to $8 billion, according to the report, which tracks deals made by institutional and accredited investors.

Facebook’s value was up 57 percent to $65 billion based on institutional investments, but individual investors on secondary exchanges were trading at an implied valuation of up to $80 billion.

Groupon was valued at $5.6 billion, which represented an 18.5 percent jump in the quarter. Twitter’s value was pegged at $4 billion, up 7.7 percent, and LinkedIn was up 43.4 percent, to $2.2 billion.

Trading in shares of these private companies has become one of the hottest investment opportunities around but has also raised legal and regulatory concerns. The nascent market is largely unregulated, leading to scrutiny by the Securities and Exchange Commission.

Raising money on secondary markets has become evolved into a crucial financing stage for growing companies that need cash but aren’t ready for the spotlight that comes with selling shares to the public.