It is unethical for an attorney to use Facebook to deceive someone. That's the short version of the lesson to be learned from the case in which the Drug Enforcement Agency just settled a lawsuit. The DEA concocted a scheme in which they usurped a woman's identity and posted on Facebook as her for several months in an effort to catch other drug offenders. It cost them (or us, the public) $134,000.
In a statement released by U.S. Attorney for the Northern District of New York Richard Hartunian, he said:
This settlement demonstrates that the government is mindful of its obligation to ensure the rights of third parties are not infringed upon in the course of its efforts to bring those who commit federal crimes to justice
It also takes into account emerging personal privacy concerns in the age of social media, and represents a fair resolution of plaintiff's claims.
If a Pennsylvania attorney were to take part in a scheme like this, they would face scrutiny and likely discipline from the Disciplinary Board of the Supreme Court of Pennsylvania, the body that oversees all attorneys licensed to practice in the Commonwealth. In an ethics opinion from the Philadelphia Bar Association in 2009 (link opens a PDF) that organization addressed an inquiry into the "ethical propriety of attorney gaining access to MySpace and FaceBook pages by using third party to make request to page owner." The Phila. Bar Association concluded that such an action would violate several Rules of Professional Responsibilty.
In a 2014 formal opinion, the Pennsylvania Bar Association Committee on Legal Ethics released its Ethical Obligations for Attorneys Using Social Media (PDF). While only advisory, and therefore not binding on the PA Disciplinary Board, it does provide guidance on best practices for attorneys who engage with social media. As with most of the ethical rules, it boils down to "if you have to ask, it is probably a violation."
An attorney who violates their ethical responsibilities may face sanctions ranging from an informal admonishment and through a suspension of their license to practice. In particularly egregious cases, an attorney may be disbarred, or banned from the practice of law entirely.