One wonders if Ava Bird is actually OK. Do people often drop off the grid like that and fail to respond to court papers &c? UK: I suspect the legal situation is a bit more complex here (it usually is), so perhaps the title should be 'most important law in the US'?
It's simpler in the UK and EU at the moment.
You want to look at the EU e-commerce directive, paragraph 43 and the definition of "mere conduit" in the appendix: http://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELE...
In essence, (and directly from the law):
"Mere conduit"
1. Where an information society service is provided that consists of the transmission
in a communication network of information provided by a recipient of the service, or the
provision of access to a communication network, Member States shall ensure that the
service provider is not liable
for the information transmitted, on condition that the provider:
(a) does not initiate the transmission;
(b) does not select the receiver of the transmission; and
(c) does not select or modify the information contained in the transmission.