More than 500m people globally have now signed up to Twitter, with Americans and Brazilians the most connected, according to a report from social media monitor Semiocast. The study claims that Twitter passed the 0.5bn milestone at the end of June and now counts 517m accounts. The US accounts for by far the largest number, with 141m of Twitter’s users coming from its home market. Brazil comes in second place with 41m Twitter users, up by almost a quarter year on year, while Japan is third with 35m users. US users are also the most engaged, posting 25.85% of all messages on Twitter, ahead of Japan on 10.6%.

While the growth is impressive, Semiocast appears to be counting total accounts, rather than monthly visitors. Twitter itself says it now has more than 140m “active users”, making it clear that this refers to members that actually use the site, rather than those who may have signed up only to come back only rarely, if at all. But Semiocast’s figures suggest that there could be more than 360m such accounts, with fake or inactive users outnumbering the real thing by more than two to one. While some of these may be accounts set by spammers, it points to a growing issue at Twitter in getting people to use the site once they sign up.

To boost this figure, Twitter has made a number of changes designed to drive up user engagement. Most recently, it launched an email digest feature that summarises the top stories and tweets in a user’s network. It also revamped its discovery service that displays trending topics and recommends users and accounts to follow to include a personalisation option. The aim is to make it easier for users to find content and accounts of interest to them and thus keep them on the site for longer and coming back more often. This is particularly important for Twitter, which has previously admitted that a large percentage of its users never tweet themselves, choosing instead to follow celebrities and new channels to keep abreast of information in real time.

User engagement is also key for Twitter as it looks to monetise its user base. The firm is relying on advertising to prove that it can make money, with initial signs showing promise. Twitter sells promoted content to brands, allowing them to advertise via promoted tweets, trending topics or accounts. And while the firm does not provide any financial detail, estimates from eMarketer suggest that it should pull in USD250m in ad revenues this year, with ads on mobile proving particularly popular. But a huge host of inactive users means a huge potential audience that the firm cannot sell advertising around. Getting people that have signed up to start using the service is key in proving its long-term success.