One big difference between a written, tangible piece of evidence such as those traditionally considered and an entry in a blog, or tweet is being able to demonstrate that the pattern of bits a lawyer wishes to put before a court are the same bits that the person actually wrote.
So while it's possible, if not easy, to determine if a written entry has been forged or altered that isn't necessarily the case with a piece of computerised evidence. For example, if the website in question passes stuff through a spell checker, or censor before publication - is that then what the person actually wrote? Also is the entry put before a court the final piece of work - complete with corrections, second-thoughts about the content and rewording/clarificatiosn, or is it merely one of the drafts that best supports whichever view the lawyer wishes to promote?
At best "evidence" from a website/forum seems to me comparable to evidence written in pencil: not permanent, easily altered (either before or after the evidence gathering process) and possibly only has the status of a doodle on a post-it, rather than a literary masterpiece from a professional letter-writer or diary-keeper, with the difference in intent and prior thought that goes into it.
If I was on a jury, I would definitely be highly skeptical of a FB entry presented to me as evidence. And if the evidence had been obtained from the prosecution using the owner's password to obtain it, I'd probably discount it unless the site itself could prove it was what the person had written and meant to have published.