I am a big fan of Seesaw! We began using it at my school from Grades 1 to 5 last year and it has had a fairly substantial impact within our school community as it helps bring parents into the classroom to see the learning that is happening. They have served us really well for student-led conferences as the students have already selected items to share and they can have a more in-depth conversation with their families about the work that they have uploaded and reflected on. Seesaw and other online portfolio options seem to be all the rage in K-12 education for many of the reasons that you state in your review, but most importantly timely, effective and meaningful feedback. The ability to use it as a communication tool is really great as well, making class announcements and notifications really easy to produce and quickly distribute to students and parents.
Seesaw can be a free portfolio service for teachers, or alternatively schools can purchase a paid subscription which allows you to add more teachers per class and a full range of school-wide analytics and management. As you caution, excessive social media activity may be related to decreased mental and emotional well-being, it is important to consider the impact of initiating a social media presence at a young age. Here at our school teachers upload videos, images, and student work from Grade 1 to Grade 3 and in Grades 4 and 5 contributions are made by teachers and students.