The online site Reference defines social trends as "Any type of activity that is participated in by society as a whole. Trends can be long-lasting or short-lived." Another site, WebRef defined it as, "A persistent change in social relations and social structure over time. Trends are the aggregate effect of many uncoordinated individual and group actions, such as bureaucratization, industrialization, urbanization, etc. Social trends also affect individuals even though individuals may be largely unaware that it affects others also in a similar manner."
So this means social trends are always happening, changing and evolving society. Humans are strange creatures. In some human interests, trends can emerge or change very quickly, such as fashion and media, even some aspects of transportation. While clothing keeps some basic constants like underwear, tops, pants, skirts, dress, coats, they can change dramatically in design, fabrics, cut, and preferred colors. New cars come out every year with new designs and innovations to increase interest and buyers. They gradually become old-fashioned. Changes in media may take a few years or decades to establish, but advertisements are constant although always changing in format and tone. So, social trends can define a historical era or become a hallmark of the present one.
Since most of my writing is in the genre of science fiction, and I tend to take historical trends to create my worlds with either approaching or past apocalypses, or how human beliefs, prejudices, and choices change worlds, I must be using trends.
Obviously, digital devices have made a mark on today's society, and I have used them in my scifi stories. Usually, I translate historical or current trends into these stories.
I'm trying to branch out into historical fiction and contemporary romance. I know I will incorporate many social trends in my contemporary, but I've already found out historical fiction is a little more tricky.