I think this is a question that's come up for me earlier in my life - and it definitely carries some implications, but it's a simple enough question at face value. It might not be a strictly "anarchist" question since there are non-anarchists who might have concerns about using social networks, but, it's a question that anarchists can easily relate to and offer advice on to some extent.
It's generally a good idea to use a pseudonym on social networking sites for the sake of avoiding unwanted attention from employers, authorities who might have a casual interest in looking into your behavior, and other nuisances.
Law enforcement agencies, however, can generally look up social networking profiles via associated IP addresses, so pseudonyms generally won't protect you there.
It's a matter of opinion whether it's "safe" to openly express anarchist ideas on the internet, I think. There are a lot of variables that come into play - the amount of manpower available to law enforcement and intelligence agencies, whether talk alone is enough to wind up with you under surveillance.
My course of action thus far has been to use a lot of different pseudonyms and never, ever, EVER discuss illegal plans or actions via social networking sites, or unencrypted communication in general (and even then, speaking in person is always the most secure way of doing things).
There's some debate over whether social networking sites are a valuable tool for communication and organization in certain cases, or if they always inevitably play into the hands of the state which is capable of accessing information on social networking sites as they please. Plus contributing to the complete absorption of all communication and organization into the virtual realm. I guess it's sort of an argument over whether the cost outweighs the benefit.
