How to cite an expert's opinion which doesn't have any title? And should I?
Believe it or not, there are guidelines on citing tweets (Chicago &amp; MLA quoted there, others may be available) and you may be able to take a similar approach. (I like the MLA approach, which is reminiscent of the very old-fashioned approach of naming an untitled document after the "incipit", its first words...)

Alternatively, you could give it a placeholder title - Smith, J. (2015-01-01). [Notes on something]. Social network site. URL - here the URL is the key element to find it and the bracketed element is just for the reader's convenience.

As to "should"... if you would be happy to use this as a useful source had he published it in something unreviewed but with a title (eg a personal blog post), then the absence of a title shouldn't really be the problem...