Facebook has reported better-than-expected first quarter profits as well as growing monthly active users.

Mark Zuckerberg's social networking giant said 2015 first-quarter profit was $512m (£341m), a fall of 20% from a year earlier. However, it noted that revenue had increased by 42% to $3.5bn

That was better than analysts had been expecting, and monthly active users increased by 13% from a year earlier to 1.44bn.

In after hours trading Facebook was down by more than 2%.

The company said that during the quarter, revenue from mobile ad sales made up nearly three-quarters of total ad sales.

"This was a strong start to the year," said founder and chief executive Mark Zuckerberg in a statement.

Despite the better than expected profits, investors are worried about slowing revenue growth - as well as increasing costs at the company, which has been spending more on its research and development costs.

Research and development expenditures increased over twofold, to $566m from $181m a year earlier.

The company has warned that those costs are set to increase, as it looks to expand some of its acquisitions including photo-sharing site Instagram, messaging service WhatsApp, and virtual reality firm Oculus Rift.