An interesting question, I would assume this would be governed by the sites TOS which usually prevents "unauthorized" use which implicates trademark issues, which in turn gives TM holders footing to petition the site to remove "offending" account names. For example, github's TOS requires "You may not use the Service for any illegal or unauthorized purpose. You must not, in the use of the Service, violate any laws in your jurisdiction (including but not limited to copyright or trademark laws)." I would presume that overall most of these would be somewhat "amicable" versus domain squabbles, but that is situation-specific.

One progression is "Amazon" on twitter which appears to have been registered by an unaffiliated developer and transitioned (rather quickly) to Amazon Inc.:

https://web.archive.org/web/20071111010348/http://twitter.co... https://web.archive.org/web/20081127003135/https://twitter.c... https://web.archive.org/web/20090414074613/http://twitter.co...

Not surprisingly, Twitter's TOS at that time was almost identical to github's current TOS: "You must not, in the use of Twitter, violate any laws in your jurisdiction (including but not limited to copyright laws)."[0]

Some may also remember firefox.com having a thanks message for the domain, which is somewhat in a similar realm.[1]

In short (insert, the usual this is not legal advice disclaimer), companies would leverage existing laws (primarily, or exclusively trademark) to point out TOS breaches to service providers. The TOS provides for "termination" of an account upon breach and the account is effectively transfered to the complaining party. Depending on the parties' attitudes there may be some delay to wind things down etc, but legally the provider can presumably immediately deactivate and transfer as they see fit (subject to the TOS).

[0] https://web.archive.org/web/20071030230315/http://twitter.co... [1] https://web.archive.org/web/20050215090602/http://www.firefo...