Google+ has launched an API that should allow developers to write third-party apps that will be able to post and read directly to the service - a move that may be sorely needed, after an independent study suggested that the number of posts to the service has fallen by over 40% in the past two months.

Google+ - also known as Google Plus - was launched at the end of June to a limited user group as the search engine giant's riposte to the successes of Facebook, which refuses to let it index all of its content, and Twitter, which has ended a contract whereby Google could index tweets and make them available in search results.

Google+ offers a method of sharing content including text, pictures and videos with people who are sorted into "circles" which are effectively walled off from each other. Posts on the service can also be given varying levels of privacy and shareability. A video service called Hangout also lets people hold collaborative chats.

The API, opened up by Google on its Google Code blog, uses REST requests, JSON output and OAuth2 for user identification.

Initially, only posts that are publicly available and shareable will be available through the API.

However Dave Winer, a strong advocate of systems that interoperate, was dismissive about the API: "Google doesn't get it", he wrote:

They need an API with one call, one that posts a tweet to their service. So people can hook up Twitter clients to Google-Plus, so the hundred million active Twitter users can post to Google-Plus from the comfort of whatever tools they depend on.

Of course it isn't the hundred million that they need, it's the hundred thousand who do all the work on Twitter. The ones that can't be bothered with a service that doesn't have basic rudimentary API support.

As he also pointed out, publicly available data could be made supported more simply by using RSS feeds (though clearly that would preclude Google Plus's API from supporting authenticated calls to private content shared with individuals).

Falling usage?

Minus slope: Google+ usage in July-September 2011, by users of 89n's ManageFlitter. Source: 89n. Used with permission.

( See the original graphic.)

Meanwhile, the service may have wider problems if the API does not get sufficient traction. An analysis by 89n.com, an Australian company that writes web-based apps, says that the average number of posts per day by people using one of its systems has fallen from an average of 0.68 per day between mid-July and mid-August to 0.40 between mid-August and mid-September.

That represents a 41% fall over the period, said Kevin Garber, 89n's chief executive. The posts were made using ManageFlitter, a tool which ties Twitter to Google+ so that people can crosspost to both.

"To date 7,280 people have linked their Google+ accounts to Twitter using our service," Garber noted. "We check these accounts for new Public posts every 10 minutes which represents a significant number of data points: 130,059 public posts have been posted in these accounts."

But after an early wave of enthusiasm, postings to Google+ through the service had waned.

"One possible explanation for this data is that the people integrating their Google+ accounts with ManageFlitter are all early adopter sorts that get bored once their new toys are not shiny anymore," Garber suggested. "However, this seems somewhat unlikely given the spread of Twitter followers across these accounts" - which its analysis showed is around 700, having peaked at an average of 1,200 early in the sampling period.

"It is also often the early adopters that drive long term adoption and to see a drop off now is not a good indication for the service's future," Garber noted.

Google has not released official statistics for the uptake of Google+ since its earnings results in July, when it said that it had 10m users.

In August, Search Engine Journal suggested that the service had reached 25m users - but said that "while the continued growth in the number of registered users is fascinating and, for Google fans, exciting, the actual amount of traffic is less so." A study by Chitika had found that "traffic peaked on 21 July and has been declining since", a fact it said was most likely due to "the end of the initial rush of excitement".

That the decline was seen during the summer, when many people may have been away on holiday, could be explanation enough. But the data from 89n suggests that the lack of interest, at least among Twitter users, has continued.

Studies into usage of Google+ have given contradictory results: in mid-July Experian Hitwise said that visits had fallen in July, while comScore said people were spending more time there.

Google+ has come under fire from some users for its requirement that they provide their "real" name to register on the service.