OK, a bit of pedantry. You don't post to their Twitter or Facebook accounts, you post to yours, just like on AB you post to your AB account. Twitter and Facebook let you tag people in your post and, if they care to read posts in which they're tagged (I wouldn't), they'll read your posts that tag them.
AB doesn't allow tagging, which is a blessing and a curse. The nearest AB gets to tagging is making someone's name the title of a thread, as in "FAO jno" or, as in this case, "Davina McCall" (note that's "Davina McCall", not "Daily Mail pointlessly advertising DVDs" ...).
If Davina Mccall was particularly masochistic then, as well as reading Tweets that tagged her, she could set up alerts for where her names was mentioned. Then she would see this thread. The truth is that she probably doesn't read either. I hope she doesn't.
The point is that there's not a lot of difference between posting in one venue and post in another. Posting nasty stuff in public on the Internet is not excused by saying "But there are hardly any visitors to that site". This site is on the public Internet and this thread can be found in Google:
https://www.google.co.uk/search?q=site%3Awww.theanswerbank.co.uk%2FMedia-and-TV%2FQuestion1586691
It's impossible to know for a fact whether Davina McCall views this thread, this site or is even a member. Maybe she likes crosswords. Posting nasty things about her on the public Internet is troll-like behaviour no matter where it's done or whether she reads it or not.