How can I get a discovery subpoena without necessarily filing a civil
You usually can't, even though that is often unfair and denies you legal relief.
There is no general civil alternative to this tool. A handful of states have pre-litigation discovery, but most do not and it is not available in the federal system. According to Professor Scott Dodson of William and Mary (quoted here by me as of 2019):
Texas allows presuit discovery for the express purpose of
investigating claims where justice or some other benefit outweighs the
burden and expense of the discovery requested. The available empirical
evidence suggests that presuit discovery is widely sought and usually
allowed in Texas, though only rarely sought relative to the total
number of cases filed in Texas. A recent survey found that presuit
investigative discovery in Texas was used primarily to ensure that the
complaint would be valid under the rules. In cases in which no lawsuit
was filed after presuit discovery was allowed, the predominant reason
was that the parties had settled. These data suggest that presuit
investigative discovery is, at least in Texas, used properly and can
be a net cost gain—early peeks at critical evidence may result in no
lawsuit ever being filed, either because the evidence is too scanty to
be worth anything other than a meager settlement, or because the
evidence is substantial enough that the defendant wishes to settle
rather than defend a lawsuit. . . .
Florida enacted a statutory requirement of presuit discovery in
medical malpractice claims under a complicated procedural mechanism.
The Florida rule is designed to deter meritless claims, as well as to
facilitate the early resolution of them, by requiring the prospective
plaintiff and defendant to share information under penalty of
sanctions. . . .
Rule 4003.8 of the Pennsylvania Rules of Civil Procedure allows a
plaintiff to “obtain pre-complaint discovery where the information
sought is material and necessary to the filing of the complaint and
the discovery will not cause unreasonable annoyance, embarrassment,
oppression, burden or expense to any person or party.” Upon a
defendant‟s objection, the court can require the plaintiff to “state
with particularity how the discovery will materially advance the
preparation of the complaint.”
Connecticut, which recognizes an independent equitable action for
discovery, is similar. To be entitled to such discovery, the
petitioner must demonstrate that what he seeks to discover is material
and necessary for proof of, or is needed to aid in proof of, another
action already brought or about to be brought. The plaintiff must
describe with reasonable detail the material he seeks.
Alabama allows for preaction investigative discovery under its Rule
27160 and provides for similar showings. A petition for preaction
discovery must show that the petitioner is presently unable to bring
the cause of action and must identify the facts which the petitioner
desires to establish by the proposed discovery. Other states [New York
and Ohio] have similar requirements.
One of the reasons for this lack of remedy is that prior to the U.S. Supreme Court rulings Iqbal and Twombly in federal courts in 2008 and 2009, a 70 year old system of notice pleading allowed you to file suit without knowing for sure that key facts were true, if you had "information and belief" to suggest otherwise, and you could use that complaint to get discovery, and dismiss the case if your educated guess that you believed to be true was wrong.
In federal court, the Iqbal and Twombly rule of pleading essentially prohibits pleading certain key kinds of facts on "information and belief". This is particularly problematic in the context of the question or any other case where key facts are in the sole possession of a defendant accused of wrongdoing.
Many U.S. states, although not all, have followed the federal rule. A list of states that had done so as of 2019 can be found in the linked law review article. The list has probably grown longer in the following three years. Also, Facebook could, in most cases, remove to federal court causing federal rules of civil procedure to apply, an issue that the American Bar Association commentators have commented upon).
Establishing that a forum jurisdiction has jurisdiction over Facebook in this kind of case could also be challenging, if that jurisdiction isn't California where Facebook has its headquarters, a state which doesn't allow for pre-litigation discovery.
The U.S. Supreme Court has, in recent years, also greatly curtailed the ability of people to sue an out of state defendant in another state's courts. See, e.g., Daimler AG v. Bauman (U.S. 2014) (overruling the rule of the International Shoe case since 1945 that a business may be sued on any subject matter anyplace it has a brick and mortar office or an agent for the conduct of its business).