There are so many social networking sites out there, its impossible to join and maintain relationships in all of them. If you are considering your SN options, consider this: in August visitors to MySpace actually declined. Facebook on the other hand continued to add visitors at a rate of 33% over June figures (according to a ComScore Study). Of the 2, Facebook is definately my SN of choice. For me the main factor for this preference is the cheese factor: I find MySpace to be too cheesey. This combined with the fact that I find Facebook more intuitive means that I spend a short time very day or so In Facebook keeping in touch with my network of contacts.
In general, there has been much debate about the longevity of sites such as these. Citing the rise and fall of Geocities (which was de rigeur in the late 90s), there has been a lot of debate that Facebook and others of its ilk will become obsolete as the masses move on to the next big thing. Only time will tell whether MySpace and Facebook will survive but in the meantime, businesses should not fail to take note the traffic that the proper use of these sites can drive right now. With almost 70 million users worldwide segmented nicely by geography, age and so on, Facebook alone offers a significant social marketing opportunity.