Facebook offers a lot of additional features which support being a member of a social network.
Some of them are:
Uploading of pictures and videos, sharing them with friends
Posting of messages on one’s own homepage, comments on them
Using the time line as a display of a chronological sequence of events
Birthday / events calendar
Notes
Listening to music
Games
"Like"
And others.
It is up to you what is considered as meaningful for your life. Also new applications certainly will arise.
It is recommended that “grandchildren” explain – as far as they know – these options.