She definitely knows how much she's loved.
Rina (her sister) tweeted about it once about how happy she is to know that she is loved even until now.
I think her absence on social media is an attribute to her quirkiness and randomness.
Even during the FIVESTAR blog her updates were inconsistent and random.
Niigaki was the one who updated the blog properly.
When Sayumi first started her blog, she was the one pushing Sayumi to spend time with her more in person, rather than keep looking at her phone.
I think it just speaks to her as a person, that she prefers face-to-face meeting rather than keeping an online presence.
And it's probably why out of all the Eririn oshi I've met, none of us has ever stop loving her.
Even if we're into other groups, or other members, or even quit the wota life altogether,
We still miss her till this day.