Sermo, the rapidly growing social network for doctors, gets a shot in the arm Monday when it announces a partnership with drug giant Pfizer. Sermo is one of the more interesting commercial implementations of the social networking concept. The doctors who join the service - and more that 30,000 have already signed up - use it to discuss diagnoses and treatments with their peers. The site makes money not through advertising - which it believes would be anathema to physicians - but by letting paying clients like investment firms and drug companies "listen in" on the doctors' conversations. As an incentive to its members, the site pays doctors $100 for highly rated postings.