Here’s a little quiz. When was the last time a significant social media network was founded in the United States? And what about a competitive search engine company? An online ad network? And what about a truly wide-ranging e-commerce start-up?

Here are the depressing answers. The social network Snapchat, in 2011. For search, Microsoft’s Bing appeared in 2009, a replacement for its Live Search. I’m drawing a blank on an ad network. With e-commerce, the answer is probably Wayfair, which arrived in 2002, and still has only 1.3 percent of the market (most retail innovation has been in niche areas, like luggage (Away) or special fashion (The RealReal)).

To put this another way: Facebook and its Instagram unit have close to 50 percent of the social media market, dwarfing all the other companies in monthly active users tenfold. Google has about 90 percent of the search market, with Bing and Yahoo dwindling ever further behind by the month. Google and Facebook also suck up 60 percent of the digital ad spend, with only Amazon moving up aggressively in that fast-growing space. And speaking of Amazon, the retail giant has about 50 percent of total e-commerce sales in the United States, with eBay and Walmart at 7 percent and 4 percent, respectively .

[Kara Swisher answered your questions about this column on Twitter.]

We’ve all known this for a long time. So it’s not much of a surprise that two American government agencies — the Justice Department and the Federal Trade Commission — have finally gotten around to looking into the dead obvious by investigating the market power of big tech companies and whether their dominance in a range of arenas has hurt competition and hindered new start-ups from forming.