Is it ok to friend your enemy in litigation? Probably not, if you're the lawyer. Remember, if you can't communicate directly with a represented party in real life, you also can't communicate through social media, e.g., requesting that they add you as a "friend." See Minn. R. Prof. Cond. 4.2. The rule reads as follows:
In representing a client, a lawyer shall not communicate about the subject of the representation with a person the lawyer knows to be represented by another lawyer in the matter, unless the lawyer has the consent of the other lawyer or is authorized to do so by law or a court order.
Id. And in case an attorney is looking for a loophole in the rule (as attorneys are known to do), Rule 8.4(a) extends the above rule to agents (secretaries, paralegals, private investigators, etc.). So lawyers shouldn't do it, and they shouldn't direct others to do it.
That said, there is generally no ethical prohibition against searching an individual's publicly available social media site for information.