Facebook could be facing an executive brain drain, with several of the company's top talent deciding in recent months to pursue new projects elsewhere.

Over the weekend Joanna Shields, the vice president and managing director of Facebook’s Europe, Middle East and Africa operations, became the latest high profile executive to leave the company since its IPO in May.

The announcement that Shields will leave Facebook to become chief executive of the UK government's Tech City Investment Organisation (TCIO) follows a string of executive departures, including CTO Bret Taylor, platform head Carl Sjogreen, and influential engineer Ari Steinberg, who previously led Facebook’s Seattle office.

Three other recent business executive departures include head of partnerships Ethan Beard, director of platform marketing Katie Mitic, and platform marketing manager Jonathan Matus.

According to TechMarketView analyst Richard Holway, Shields’ timing is interesting given that the lock up on shares given to staff at or before the IPO expires on 14 November. He said that many believe there is a very testing period ahead for Facebook.

“Facebook’s accounts at Companies House reveal that around 90 UK staff were handed 7.3m shares – worth about $140m even at today’s depressed share price,” said Holway. “It is not revealed how many Shields received but as the most senior person one would expect it to be sizable.”

Holway said that this might explain why Shields was willing to accept an annual salary of just £115,000 at TCIO, as reported by the Telegraph.

“One suspects that she is not making the move for the money,” he said.

It seems that executives are a lot more willing to leave Facebook now than they were a few months ago, when the social network was still thought to be the hottest, fastest growing, most exciting company in the world.

The company's disappointing IPO, which has seen its stock price fall from $38 to under $20 in five months, may have persuaded some employees that it was time to move on.

There is no doubt that Facebook is as popular as ever with users, with more than 1 billion people now logging into the social networking service each month – almost one-seventh of the world's population.

However, Facebook may need to make some changes if it is to retain its top talent and prevent high-ranking managers from being lured away by more challenging and rewarding opportunities with smaller companies and start-ups.