Metaphysical reductionism about X holds that X can be reduced to Y, where reducing X to Y means identifying X with Y.
Y is generally something that the reductionist is already comfortable believing in, while X is something previously thought of as more metaphysically problematic or suspicious (hence the label ‘reductionism’, rather than just ‘identificationism’). A reductionist might, for example, seek to identify romantic love with a particular brain state -- call it B -- motivated by the thought that brain states are relatively well-understood, at least compared to love.*
Metaphysical reductionism is not the same as metaphysical deflationism. Metaphysical deflationism about X holds that X is not a big deal, metaphysically speaking. One can be a reductionist, for example identifying love with brain state B, without being a deflationist, i.e. without denying that whatever love gets reduced to (brain state B in my example) is metaphysically substantive.
A deflationist about love might claim that our concept of love does not track anything significant, and that although as a matter of fact we happen to classify situations using this concept, we might have used others without missing out on anything of metaphysical significance. One version of this kind of view can be based on the thought that our concept of love is disjunctive, like the concept of being-either-a-railway-company-or-a-flying-ant. It’s not that there’s no such thing as being-either-a-railway-company-or-a-flying-ant, it’s just hard to see how kind of disjunctive property could count as particularly metaphysically significant. Other kinds of deflationism might be motivated by the thought that although there is a lot of talk involving ‘love’-vocabulary, which on its surface looks like a description of the world, this 'love'-talk is really some kind of projection of the speakers' attitudes, rather than any kind of description of the world (even a disjunctive description). On this view, it would be metaphysically misguided to seek for something in the world that corresponded to 'love'-talk.
Neither reductionism nor deflationism is the same as social constructionism about love. A social constructionist about X claims that X is socially constructed, i.e. created by a society or culture. This is not per se a reductionist claim: for one thing, it doesn’t identify love with anything in particular, and for another it need not be assumed that social constructions are in general less metaphysically problematic than love is. Nor is social constructionism necessarily deflationary, since it is not a commitment of constructionism that social constructs are metaphysically non-substantive.
* To be clear, this thought would be controversial!