Twitter co-founder Jack Dorsey invested in Foursquare

Twitter just closed a massive funding round that reportedly has given it a billion-dollar valuation. Meanwhile, co-founder and chairman Jack Dorsey is making investments of his own: he's one of the undisclosed angel investors in geolocation start-up Foursquare, quite a few sources have told CNET News.

News of the New York-based Foursquare's venture round, led by Union Square Ventures, leaked earlier this month via an SEC filing. A source with knowledge of the deal's terms said that about $200,000 of that $1.35 million in funding was taken up by the angel investors, including Dorsey, but that there are quite a few hats in the ring so none of them has a particularly huge stake in the company.

Foursquare's executives have chosen to keep the names on the list quiet.

Twitter and Foursquare already have an investor in common: Union Square Ventures, which participated in Twitter's Series A and B rounds but sat out on the most recent one. AllThingD's Peter Kafka speculates that a $100 million funding round may have been out of the question for Union Square, which specializes in early-stage investments.

Jack Dorsey, meanwhile, was Twitter's inaugural CEO, but stepped down in favor of fellow co-founder Evan Williams, a more seasoned tech industry veteran, about a year ago. Dorsey remains an important face of the company even as he works on his next company, reportedly a mobile payment gadget start-up code-named "Squirrel."

A source with knowledge of New York's start-up scene says that Squirrel's real name will actually be "Square." No relation to Foursquare. We think. (Dorsey wasn't immediately available for comment.)

Side note: Squirrel, or Square, or whatever its final name is, may be headquartered in New York rather than the Bay Area. That may have been what set into motion some erroneous rumors this month that Twitter itself would be relocating to New York. Twitter's definitely hunting for new office space to house its rapidly growing workforce, we hear, but it's staying in its home city of San Francisco.

But back to Foursquare. What's interesting is that Twitter's application program interface (API) will soon expand to include geolocation data, something that could potentially make it compete with the core business of Foursquare--a tiny start-up that was basically built from the ashes of ill-fated Google acquisition Dodgeball and launched this year at the South by Southwest Interactive Festival. Dorsey's investment is obviously personal, not on behalf of Twitter, but now he's got a stake in both companies' success.

UPDATE at 8:26 a.m. PT: This post was updated to clarify that the name of Dorsey's new start-up may be "Square."

UPDATE at 12:16 p.m. PT: Business Insider reports that veteran angel investor Ron Conway is also one of Foursquare's numerous individual investors.