News editor Jussi Pullinen said the Sanomat set up the channel as part of its strategy to reach younger readers.
"We noted that the activity of young people has gone down on Facebook, even here in Finland. They are still there, but they are not as active as they used to be, so we got thinking about WhatsApp.
"It spiked in Finland last year or so, its growth has been phenomenal here," he said.
Helsingin Sanomat runs its WhatsApp service as a daily newsletter under its Nyt ('now') branch, the section looking at events, pop culture, music and the internet, with a more edgy approach to news.
Helsingin Sanomat has 2.4 million unique weekly visitors, and about 500,000 of them visit Nyt.
The daily newsletter features three or four news headlines, including local stories, entertainment, and commentary on national politics, and is finished off with tips about events or recommended film listings, almost like "an agenda for the day", said Pullinen.
Nyt sends just one message a day, in the afternoon, but its subscribers often reply and chat with the reporters, asking questions about upcoming events or sending photos.Based on what people write to us, I don't think that they would really appreciate as much an impersonal, hard news broadcast.Jussi Pullinen, Helsingin Sanomat
"It's a really direct channel of communication, the messages are very personal, they're very different from the stuff that we did via email for example. It's more direct, more personal, and more chatty."
As the service is only two months old, the Sanomat is looking at ways to evolve it, as well as tallying up what has worked so far, as noted from the subscribers' feedback.
"We've been asking people what they want and what they like, and I think there's a ratio there, it's like two to one.
For every hard news story included in a message, he said, "you need to have two that are a bit more 'lifestyle'".
"Based on what people write to us, I don't think that they would really appreciate an impersonal, hard news broadcast as much."
But while the Sanomat might not have plans to increase the number of hard news headlines it sends out to its current Nyt WhatsApp subscribers, or to start a spin-off service exclusively dedicated to news, Pullinen said the readers still need to feel like they are learning something from the newsletter.
"That's what I've learnt from what they're writing back, they don't just want cat features, they do want news and they want to get informed but in a way that's more personal."
Chat apps have become a channel for news distribution for an increasing number of media outlets in the past year. The Oxford Mail started a WhatsApp service last year as a more direct way to connect to its readers, and now has over 1,200 subscribers.
The BBC has been experimenting with WhatsApp and Line, and set up a WhatsApp service in West Africa in October, with news bulletins, often in audio format, about the spread of the Ebola virus.
But the Nyt newsletter is text-only, as WhatsApp doesn't allow users to add multimedia elements to a single message and requires images and other files to be sent separately.
"The last thing we want to do is be spammy or send news photos that are then uploaded to their camera rolls." Pullinen explained the Nyt team wanted to be as non-invasive as possible with their messages.
He added that the next step for the Sanomat is to build a community on WhatsApp.
And while story ideas received through the chat app have so far been limited to small tips such as the opening of a local restaurant for example, establishing a conversation on WhatsApp is becoming a priority.
[We want to] build a channel where people can be in contact with us, and they can feel like they are getting something from us in return and they are not just subjects that we are broadcasting to," he said.
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