As social-networking sites such as MySpace and Facebook grow by leaps and bounds, they attract a growing number of spammers and advertising - and consequently drive away some users altogether.

The big social-networking sites are facing a dilemma: Each needs a community large enough to support it, but if the community grows too large it can alienate some members, causing them to leave, writes the Wall-Street Journal. Both sites lost visitors in September, according to Nielsen/NetRatings. Both sites say the drop is seasonal.

Advertisers as well as spammers are exploiting the "friend" feature of MySpace to send thinly disguised commercial messages to members. Though both sites became popular by connecting users online, more and more members are turned off by how easily they can be reached by those they might not want to have contact with.

The Wall-Street Journal points to the example of a female student approached by a fellow student asking for her number. Though she declined, minutes later the student had found her on Facebook and attempted to add her as a friend.