You don't need FB links to be dofollow. Firstly, you'll get credit for having the link anyway (trust rank, authority domain etc), and secondly you'll get direct traffic from FB. Remember FB is bigger than Google now so potentially could drive a ton of traffic your way.
I don't think that it is possible to turn the nofollow links of facebook in dofollow but the nofollow links from counts worth because they are helping you to drive good amount of traffic from facebook.