Nokia is set to announce today that it has acquired Plum Ventures, the San Francisco “microsocial networking” start-up, for an undisclosed amount, sources said.

UPDATE: Nokia confirmed the purchase, but gave few details in a press release.

Nokia has Acquired Plum

ESPOO, Finland and SAN FRANCISCO, September 11 /PRNewswire-FirstCall/–Nokia (NYSE: NOK) and Plum today announced that Nokia has acquired certain assets of Plum Ventures, Inc, a privately held company which employed approximately 10 people with main offices in Boston, Massachusetts. Plum will complement Nokia’s Social Location services

The acquired Plum assets will become part of Nokia’s Services unit.

The small company was founded several years ago by Hans Peter Brøndmo as a social-bookmarking site. He then shifted its focus, trying to make a business in the places big social networks ignore.

As part of the deal, Brøndmo told BoomTown that he would be moving to Berlin to work at a Nokia unit there, but that the service would continue to operate as it has been.

As I wrote when I visited Plum in May:

“Sites like Plum are what many like to call microsocial networking, used by people or Web sites who want less the overwhelming experience that the large social networks have become and more an ability to create with a smaller group.

It’s hard–though not impossible–to do this on Facebook, and Twitter is all about broadcasting, so it’s interesting to look at those working on sites for small and private social groups and the various approaches they take.”

This kind of ability to link more intimate and closer-knit groups, like families, is likely what attracted Nokia (NOK), the Finland-based mobile phone giant. Plum is also used by third-party Web sites, such as travel search site Mobissimo.

It was backed by Levensohn Venture Partners and Vulcan Capital.

But despite Nokia’s size and power, the company has been has been struggling to compete with the rising popularity of highly interactive smart phones like the Apple (AAPL) iPhone.

Here’s a video interview I did with Brøndmo on my visit to Plum: