Twitter has today announced on its blog that there are 1 million registered Twitter apps, built by more than 750,000 developers. This is a huge increase over the only 150,000 apps that were plugged into Twitter a year ago. This is also a marked increase of 100,000 from numbers that were released by Twitter in May. Twitter says that a new app is registered every 1.5 seconds.

Twitter has also announced a brand new Twitter Developer site that developers that integrate Twitter can use to talk to Twitter team members, converse with fellow developers and access resources.

In the posting Twitter notes that more than $500 million has been invested in companies that are using the Twitter ecosystem and that more than $1B has been spend on acquiring companies that do so. Twitter says that this is indicative of the opportunities that entrepreneurs have to build successful businesses as part of the Twitter platform.

Twitter goes on to provide several examples of companies that are using Twitter to build or enhance their business. Among these is social statistics company Mass Relevance, which powered the recent Twitter Town Hall where President Barack Obama answered questions curated live from Twitter. There are companies which build their business completely around Twitter, like Poptweets, an iPhone trivia game as well as companies like Radian6, a social media analysis company that was recently acquired by Salesforce.

The announcement of the new site for Twitter developers appears to be an effort by Twitter to ensure companies using its platform that they can build with confidence. The social network wants people to take full advantage of the service in their products without the fear that access to it may be blocked in the future due to a change in policy. This posting seems to address mostly companies that are using the Twitter API in ways other than as standalone clients or “hole fillers.”

This would fit with the advice that Twitter gave to developers to stop building clients that just access Twitter. This advice caused major ripples among the community of developers making third-party clients for Twitter, who felt threatened by the announcement. Many of Twitter’s features like retweets actually grew out of third-party client features and user behavior. Twitter officially responded to developers in an attempt to allay fears that alternative clients would be blocked from the service entirely. In that response, Twitter’s Ryan Sarver said that he believes that an ‘open dialog’ is important. Perhaps this new site dedicated to Twitter developers will help give the services policies regarding API’s and applications some transparency, helping to rebuild the developer relationship.

Regardless, the numbers presented here are seriously impressive and it’s good to see the Twitter ecosystem growing well. But it’s worthwhile noting this is still on a much smaller scale than rival network Facebook, whose users install some 20 million applications every day.