By "social activism" most people mean demonstrating, protesting, or otherwise attempting to influence immediate events in society, and still under capitalism. These attempts to reform capitalism have a very long history: as long as capitalism itself. We call these actions "reformism".

Organizations which claim to want socialism, and which also promote reforms, ignore socialism and spend their time working for reforms.

The Companion Parties of Socialism, in the World Socialist Movement, are socialist parties. They promote socialism because that is all a socialist party can promote.

If you find a "socialist" party promoting "social activism," you'll have found a non-socialist party.