The single-use app, which pings "Yo" notifications back and forth between its users, already had more than 1m users before its latest expansion.
Developed in Israel, the app is backed by $1.2m of seed funding, with its developers hoping to prove that it is more than just a novelty app championed by journalists and Silicon Valley execs for a joke.
"The past few weeks have been incredible, with millions of users discovering how simple it can be to stay in touch with just with a Yo," said creator Or Arbel in a statement. "We are going to keep bringing that experience to as many users as possible and the new version is an important step in that direction."
There are plenty of sceptics, but Yo is already showing potential when linked with other internet services.
For example, IFTTT has launched a dedicated Yo channel to link the app with other gadgets and digital services, including using it to control home lighting (if the lights are from Philips' connected Hue range) and calling a user's phone to help them get out of a meeting.
Meanwhile, a Yo hackathon in late June saw developers coming up with other uses for the app, from notifications of server downtime and bookmarking songs playing on the radio for later listening, through to a toaster capable of yoing its owner when their toast is ready.
The hackathon has also spawned a charity campaign, YOTHEWORLD, which is trying to raise money to fund surgery for a Haitian child with a rare heart defect.
The Windows Phone app, IFTTT channel and hackathon cap an eventful month for Yo, although it has not all been good news for the fledgling startup.
Yo was hacked in mid-June by a group of students, who claimed to be able to access users' phone numbers as well as sending notifications from any account. Yo has since patched that particular security hole, hiring one of the hackers to help.
guardian.co.uk © Guardian News and Media Limited 2010