That's hard to call, though I see a few dynamics affecting the outcome.
1. Cost matters. In communications, a low- or zero-marginal-cost solution almost always has an advantage overy any marginally-priced solution. I'm quite confident in predicting that micropayments will not be the solution here, though some time-based all-you-can-eat solution might work, more likely at an annual or decadal scale than a monthly (or shorter) one.
2. New media tend to be adopted based on attractive founding cohorts. People are drawn to communities that are selective, exclusive, and/or generally represent a higher socio-economic-political status than they have. If you look at the history of communications, going back to speech and writing themselves, those with facilities in spoken or written language were seen as attractive and appealing.
Early Usenet, and later Facebook, both developed among selective-admissions research/prestige universities. I don't think this is at all a coincidence, and each network could be contrasted with more general, less-selective emerging or extant networks of the time, say, BBSes, Prodigy, AOL, or MySpace. See dana boyd who's also pointed this out.
(Corollary: do you want to create a Facebook killer? Create a billion-dollar fund and and seed $10m to each of 100 new networks within an aspirational community, reserving the right to merge any successful networks emerging from those into an aggregate, aspirational community. Google came close to this formula for G+, but erred, contrary to a great deal of commentary, in opening up the network too soon, and most especially, to the kiss-of-death native Google community of marketers, SEO, and advertising. If it had stuck to tech and STEM for a few years, it might have created that vital community. In my opinion.)
3. The directory is a critical element. In SMTP email, this includes DNS and local elements. DNS itself is a stumbling block for many people to self-hosting services, and it really wasn't meant to scale to billions of individual domains. There are hierarchical structers that could do this, including DNS hierarchies (domain, subdomain(s), host, user), or X-500 or LDAP, but which have not been widely adopted. Microsoft's Active Directory (an LDAP variant) exists, but is proprietary. I'm not sure what the solution here looks like, but it's actually among the key sticking points.
4. Support and privacy of metadata. The history of the standard RFC 822 message header structure is an interesting one, and dates to the early-20th century development of the inter-office memo. See Kathy Yates, "The Memorandum as Management Genre" (http://www.ismlab.usf.edu/dcom/Ch6_YatesMemoMgtCommQtly1989....). Among the surprises I found there, "In-reply-to" actually pre-dates" the "From", "To", and "Subject" fields -- creating chains of referenceable communications matters.* A key problem of current SMTP messaging is that these metadata are among the most useful to any surveillance system, capitalist or state, and yet are not protected by extant content-encryption systems. At the same time, some access to metadata may be critical for filtering purposes.
5. Various forms of trust and graduated trust-delegation. This applies at various levels. There's a level of trust which must generally be granted to message transport systems -- software, hardware, channels, protocols. Filtering against spam or unwanted email may involve some level of disclosure of either identity or contents. (It's likely that spam won't generally be (strongly) encrypted making filtering of it more viable, but this remains to be seen.) The ability to layer trust and encryption such that access, approval, and/or vouching may occur at various levels in a limited fashion could help.
JoAnne Yates' "The Memo as a Management Genre" traces the origins of business writing, including the emergence of standard fields -- "From", "To", "Subject", "Date", and "In reply to", which actually predates the other four -- go Mutt and threaded discussion! The influences on subsequent communications formats, particularly Email and Usenet, aren't mentioned, but are obvious.
Letter-writing itself, as a widespread practice, is fairly new, it relying on post offices, stamps, and paper, and above all, literacy. The forms of letters written in the 19th century were very much defined by guidebooks and conventions.
Business writing itself is also highly formulaic, though it dropped much of the verbosity (and social grace) of earlier forms, discussed in this essay.
Messaging-as-technology, the protocols, conventions, forms, usage, and economics, are all mentioned, in particularly the induced demand created by easier communications, storage, and retrieval (or reversal as practices were limited or discontinued).