There are many. Email is a particularly notable example. Luckily, the technology sector wasn't run by "entrepreneurs" back when email was invented, or we would have wound up with walled gardens there too (Hotmail users can only email other Hotmail users and contacts can't be exported, etc).
But it was. Back then, you had "online services" - Prodigy, Compuserve, GEnie, and AOL. And for the most part, users on one of those services could only mail other users on the same service.
Email and the Internet won because the sector was competitive enough that no one service had enough market clout to not offer e-mail. Once some people got e-mail accounts, other people wanted to e-mail them, and threatened to move off the services that didn't support it in favor of services that did. That scared the online services, so rather than lose, they all started offering e-mail themselves, until they lost anyway.
If you want open social-networking platforms, the solution is to make the market competitive enough that no company can afford not to be open. Usually that'd involve choosing to use the smaller players until they're big enough that no one company is gigantic.
We did have walled gardens. I had accounts on Compuserve, bix, cix, MCIMail with a college account that did mail with bang path addressing over uucp (this is in 1986/7).
Most of the UKs email over uucp came in via the ukc gateway and IIRC I think I was charged something like 4p/1K for email that was international at that point. From memory my usenet feed came via a US bank in the City and was over a telebit trailblazer modem.
By 1990 I'm pretty sure that cix,MCIMail and Compuserve all at least had gateways to SMTP mail and within a few years it all became more more transparent and free.
Interesting point, although I don't consider email a platform like I do Facebook or Twitter. While there's no central database, there's a means of determining where the right server to talk to lives attached to the username.
We'd need some sort of DNS-for-handles in order to implement something similar for a new protocol, unless you want your handle tied to a domain (in which case you've just remade email with a RESTful API and a length limit)
