In case commercial social networks will not protect freedom of speach
there is always option to use mailing lists.
In fact I can imagine Twitter or Facebook as web based mail client
which aggregates urls for messages publicated on anywhere in Web.
Everbody who observes somebody will receive email with link to message.
Likes and comments are also mails (the can even be signed).
Webserver which serves given message will receive like and comment
mails and presnet it. Yo can see source of mail e.g for like AND verify
that like was realy given by ckecking GPG signature of sender.