Digg has started adding nofollow to some of its links - the idea it to stop people spamming the service to get links back.

According to its blog: "We've added rel="nofollow" to any external link that we're not sure we can vouch for. This includes all external links from comments, user profiles and story pages below a certain threshold of popularity."

So, what's the threshold of popularity?

Orignally, from my quick hunt around, I reckoned it was somewhere above 200. However, some sites appear to be trusted and need fewer.

And Ben Hazell tweeted to say that it varies by section: "you can get followed with 60 in Lifestyle, but no followed with 200 in news".

Final update: Adam Sherk seems to have nailed it. When a story goes popular, the nofollow is removed. The number of Diggs needed to go popular varies by category.

Here are some screenshots to demonstrate. The pink links in the screenshot are nofollowed. Let me know if you spot a different patttern ...

Below 200 Diggs: most news links nofollowed

A few trusted sites get a followed link below 200ish - but nearly all are nofollowed in the news section. The threshold is much lower in lifestyle ...

Below 200 Diggs, nearly all links are nofollowed

Around 220ish Diggs

At around 220, many sites start to get a followed link whichever section it is - which is because they will have achieved Digg popularity.

Around 220 Diggs, many sites get a followed link

Lifestyle section

You seem to need far fewer Diggs here to get a followed link.

Digg threshold is much lower in lifestyle

Some sites are more trusted than others

New Scientist seems to need not very many Diggs for a followed link - the NY Times a whole lot more - but this may be related to the section the stories are in. It's whether the story was made popular that accounts for this difference.

NY Times doesn't get a followed link with 149 Diggs

New Scientist gets a followed link with just 58 Diggs

Sites are, however, treated differently for different stories

The Telegraph has two stories with quite different Diggs - the one with 261 Diggs is nofollowed, the one with 214 Diggs is followed. So the nofollowing clearly isn't just done on the trust rating of the site plus the number of Diggs - again, this might be a section thing. It's whether the story was made popular that accounts for this difference.

Telegraph story with 214 Diggs: followed

Telegraph story with 261 Diggs: nofollowed

The Daily Telegraph has had more stories submitted to Digg, the social news website for sharing content, than any other UK newspaper site.

It lost out to Times Online in StumbleUpon, but the Telegraph has had more stories submitted overall, and has had more stories that made it to the front pages of Digg.

Submisions to Digg

The table shows the number of  results at Digg, using its 'URL only search' to show how many stories were sumbitted from the URLs of the main weekday papers' sites.

Clicking on the links will take you the search results at Digg for all stories. The table is, however, ranked by stories that made it to Digg's front pages (ie popular stories on Digg).

All 8 sites have a Digg button on their news stories (unlike with StumbleUpon, where the Times and Sun had most submissions, despite not haveing an SU button on their pages).

Diggs vs visitors

Digg numbers will be affected by visitors. The table belows shows the ABCes figures for visitors in December 2008 (the FT doesn't publish this figure).  The final column shows the number of front-page Diggs per million visitors.

The figures could be misleading - as it compares every Digg ever made with traffic levels in one month. But it seems to show that visitors to the Telegraph and Mail do a much better job at getting people to Digg their pages.