How to store dynamically built lists in database in a natural way so that they are just ready for reads?

How to store dynamically built lists in database in a natural way so that
they are just ready for reads?

For a social network site, I need to store dynamic lists for each entity(&
millions of such entities) which are:
frequently appended to
frequently read
sometimes reduced
lists are keyed by primary key
Need to consider for easy scaling & administration by small team &
starting out with just 1 or 2 medium sized VPS(s) & adding more as data &
load grows.
I'm already storing some other type of data in mysql but for these storing
dynamic lists, MySQL is not anyway ideal. I know that I could store my
data in an RDBMS in such a way that using JOIN(s) I'll be able to generate
those lists but I'm trying to avoid JOINs altogether to make the system
easy scalable & instead store prepared lists in a natural way. Since lists
are of variable length, storing list items as columns within a row is not
possible.
So how do I solve this ? Should I use another database for this data,
probably the one that provide adding variable no of columns to keyed by a
primary key. Something like Cassandra ?