Hackers broke into the app, which measures the influence of Twitter users, and used it to spam the Twitter pages of people who enabled the service.
Grader users found themselves sending a Twitter message that read 'Biz Stone Promoting Twitter in 2006' and contained a link to a newly registered web domain that hosted a video of Twitter co-founder Biz Stone.
The hackers appear to be trying to improve the search engine ranking of the domain Seonix.org, an online money-making site, which was registered this week, said Rik Ferguson, a security researcher with Trend Micro who blogged about the incident.
The founder of Grader.com's parent company, HubSpot, said his company was blocking the unauthorised messages, but he could not say how the compromise occurred.
"All indications are that security on one of the Grader.com applications (Twitter Grader) was compromised," said Dharmesh Shah.
"As a result, an unauthorised third-party was able to post tweets on behalf of some of our users."
"We have updated the system to not allow unauthorised tweets to be sent out anymore, and are working furiously to research the issue further and make changes such that it doesn't happen again," he said.
"Security issues are never fun and we hate that this happened. Our apologies to all of our users and those that have trusted us."
The issue was "totally our fault," he added.
Shah's own Twitter account sent the Biz Stone spam message, as did the account used by Grader.com.
Twitter users caught a break this time because the Seonix.org website is not malicious, according to Ferguson.
If they had wanted to, the hackers could have tried to install unauthorised software such as a Trojan horse program on the machines of anyone who clicked on the Seonix.org link.
It's not clear how many people use the Grader.com service, but the company's Twitter account is followed by more than 50,000 people.
The hack shows why hackers are increasingly interested in social media and the applications that work on sites such as Twitter and Facebook.
"If you can pick an app that has a lot of users and find a way in, then it's a real big bang for the buck," Ferguson said.