The negative part of "virtual friendships" is the lack of eye-2-eye conversations involving body language, and that is 2-fold.
On one side a person can give an opinion via the keyboard and be construed as being "wrong" and an opinion of that person is built. If that same conversation is held f2f you may understand the process in which the opinion was developed.
At the other end is the cyber poster who goes behind the backs of others to spread their "ill-will" of someone and that person will never have the chance to defend themselves and will be portrayed as a schmuck from then on.
IMHO, While this is true of all friendships, the varying levels of honesty/sincerity, is the negative side of "virtual friendships", and even worse is the potential of character bashing that has increased by way of the "backdoor" poster.
Because of our common interests and the "maturity" of this board I don't think this is a widespread HSBBW dilemma, however with the advent of Facebook and similar sites I feel that virtual friendships are potentially dangerous especially with the young websters.