Research firm Strand Consult published a note that highlights the immense opportunity available to Facebook, as the company, which is on target to hit the one-billion user mark later this year, is struggles to boost its average revenue per user.

The report suggests that Facebook could make between $300 million and $800 million from a VoIP service annually (that’s roughly £200 million and £500 million) based on Skype’s last disclosed average revenue per user and reckons that the company is more likely than anyone else to come up with a Skype killer.

There’s a few footnotes that the Strand Consult doesn’t include in the press release they sent us though. Firstly, Skype and Facebook already have a long running agreement and Facebook has tightly integrated Skype’s VoIP (including video conference) in its own offering.

Vice-versa, Skype now has a Facebook window within its desktop client, giving the social network a vital presence on the desktop. In addition, it is worth noting that Microsoft has been a minority shareholder in Facebook.

Another interesting point to bear in mind is that an HTC-built Facebook smartphone is rumoured to be in the pipeline with a launch scheduled for next year, one that could even be based on Opera.