The Facebook Mentions celebrity app has now been made available for download on iPhone in more than 40 countriesm making it possible to follow celebrities like William Shatner and Robert Downey, Jr. who have "verified" Facebook pages.

The app – which had a limited release in late July – helps high-profile people see and respond to messages posted on the social networking site about them.

Mentions, considered a rival to the way Twitter alerts its users whenever they are mentioned by other tweeters, is available to anybody through the iTunes store.

However, while it can be downloaded by anyone, it is unusable unless you have a verified Facebook page or profile. You can get a “Verified” status – identified by a blue checkmark on the page – by invitation only from Facebook, which has not revealed how it identifies someone as a celebrity.

The new app tracks trending topics and allows celebrities to engage in live Q-and-A sessions with fans. The app also simplifies the posting of photographs, video and remarks and makes it easier for a person to join in the conversation on trending topics.

Canadians such as Liberal Leader Justin Trudeau and rapper Buck 65 have verified Facebook pages. According to Canadian Facebook spokesperson Meg Sinclair says there are tens of thousands of verified public figures globally on Facebook.

One such notable figure who has been using Mentions is Canadian actor William Shatner of Star Trek fame who began using the app when it had an early rollout.

Shatner took to his Tumblr account to rant about the new app:

“When you install the Mentions App you cannot proceed further until you follow another one of their other ‘celebrity’ accounts. The first person on the list I was given was George Takei (rolling my eyes.) I ended up choosing Robert Downey Jr. to follow and then I hid his posts (sorry Robert!) I think that is a big flaw in the set up. If this app is for celebrities then WHY force them to follow another celebrity in order to set up this app?”

He concludes the app "seems to be ill conceived."