It started by putting up optional sign up forms with a cleverly hidden opt-out link. Over time, sign up became mandatory. Now I can only share files with other Dropbox users. That's not what i signed up for. I understand that Dropbox needs to make money. Today, they do that by showing investors that they are growing. One key measure of growth is the number of user accounts they have. So, it makes sense to make every effort to get people to sign up. But, in this case, they are not gaining users through the value of their service – they are using the value of my relationships and my data to force people to sign up. Not cool, guys.

And they provide a solution :) There is also a lot of open source software that can be used.