Pinterest opens site to all, stops invite-only policy

Image-based social networking service Pinterest has relaxed its sign-up policy, opening the site to all.

Previously, people needed invites to become members and "pin" online images onto its virtual "boards".

Launched in 2010, Pinterest became the fastest-growing site ever, passing 10 million users in nine months.

Although it has tried different money-raising techniques, the company has not yet disclosed a commercial plan.

"They're one of the main sites that serve the so-called social collectors, people who like to curate information across the web," said Forrester Research analyst Darika Ahrens.

"What remains to be seen is what happens if they start to commercialise - how they're going to do that and if they do it in a way that's not going to alienate their users.

"So far, they have tried a number of different methods such as using monetising links that bring in a revenue share, but they stopped that."

One social media analyst told the BBC that having invites had been part of Pinterest's marketing strategy.

"This tactic of adding exclusivity upon launch of a social network is not a new thing," said Rob McNair from mycleveragency.com.

"Google+ adopted [it]… creating a people-referral engine and as a result built pent-up demand from the everyday user that wanted to give it a try."

Fashion and recipes have quickly become top topics on Pinterest, and women outnumber men on the site.

Its number of users grew hugely over the past year, according to a digital media analytics company Comscore.

It said that last June, there were 1.2 million unique visitors globally - but a year later there were 31.2 million.

In February, the company had to address concerns about users "pinning" copyrighted material.

It now allows websites to opt out of being featured on Pinterest. To do so, sites can block their content by adding a line of web code.

If a Pinterest user attempts to share images or other material from a site with the "nopin" instruction, a message is displayed: "This site doesn't allow pinning to Pinterest. Please contact the owner with any questions. Thanks for visiting!"