A previous comment noted that sodium compounds are generally soluble and would not be strongly adsorbed on malachite and anglesite.
However, the other compound formed in the reaction between Na2S and a copper or lead salt would be CuS or PbS, both of which are very insoluble. Three cases can be visualized: 1) a sodium ion approaches the Pb or Cu salt and this aquated sodium ion forces off a lattice-bound dication, or 2) a sulfide ion approaches the Pb or Cu salt and either deposits on the crystal surface, forcing a nearby OH-, CO3-- or SO4-- to leave in order to maintain electrostatic neutrality, or 3) the low (but not zero) solubility of the Cu and Pb salts allows precipitation of PbS or CuS as tiny crystals which more frequently than not become bound to the original salt by epitaxial considerations.
In the case of CuSO4 applied to ZnS, ZnS is about 20 times more soluble than CuS, so an exchange of cations will occur, with the copper going into the insoluble phase.
Case 1 seems far-fetched. Cases 2 and 3 are similar, and may occur simultaneously. The key is that the insoluble product is what sticks to the material that you want to float. And the surfactant that forms the bubbles sticks to the activator (CuS or PbS).
It's interesting that ZnS is not easily flotable. ZnS in water may acquire an aquated layer, that is, it is activated by water, which is not selected by the surfactant, so it sinks. Both the copper carbonate and lead sulfate would seem to have surfaces more compatible with water than their sulfides, too, so converting the surface is required.