I wouldn't say it's to contact her, as such. Yep, she is obviously checking it now, but it's been a mainly dead hashtag for a while, apart from a few people taking pictures of her book.
It's to try and get the word out there that all may not be what it seems.
I was just thinking, if you hashtag search #MeghanMarkle (first celebrity that came to mind for some reason!) on Twitter, there are all sorts of opinions and articles - some positive, some negative and some neutral. I'm sure Meghan has better stuff to do with her time, like work with important organisations and raise awareness of issues, than to block anyone on Twitter that posts something she doesn't like