Four out five social network users have received unwanted 'friend' invitations, messages or postings over the past year, new research suggests.

Messaging security firm Cloudmark said that this rise in 'spam' threatens growth and membership retention at the popular networking sites.

The attacks work in much the same way as traditional email spam in that they target users with unsolicited product messages or attempt to redirect them to a phishing site or one hosting malware.

Users have reported receiving an average of 64 unwanted communications in the past 12 months, and 37 per cent have noticed an increase in the number of unwanted messages in the past six months.

"The results of this survey should be of concern to social network operators and users," said Neil Cook, vice president of technology services at Cloudmark in EMEA.

"Social networking sites need to be concerned about the proliferation of spam and phishing attacks and the impact it could have on their ability to grow and retain members.

"Social networking providers must address the security issue head-on or risk declining usage and revenues."

Although not nearly as big a problem as spam email, two-thirds of users said that they would consider switching to another social network if they received a lot of unwanted messages.

Figures suggest that nearly half of the online adult population has at least one social or professional networking website account.

Facebook and MySpace collectively host more than 170 million monthly active users worldwide, according to Forrester Research.

"Consumers need to take the same precautions they have adopted in other forms of online communications, including never responding to unwanted messages and never posting personal information that could lead to identity theft," said Cook.