I would be tempted to add: "great company". Or some variant.

I don't think it means a socialite. A socialite is a person who is prominent in fashionable society, who spends much of his/her term going to smart parties and providing copy for gossip columnists: someone like Paris Hilton.

But a "mujer de tertulia" means a woman who excels at conversation, who is in her element in tertulias, which, before the word degenerated into meaning a vapid chat show, meant a cultured, quite often learned, albeit informal, gathering, often held in a café, where the object was to talk about something, not just idle chat but real conversation, developing and exchanging ideas. I think "mujer de tertulia" and "hombre de tertulia" imply first and foremost skill in the art of conversation. Not just chatty or glib, but actually having something to say. And in company: tertulias are essentially collective. So "great company" would be along the right lines.

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Note added at 50 mins (2011-02-17 15:17:14 GMT)
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"Es un hombre de tertulia, amigo de sus amigos, gran conversador de lo humano y lo divino."
http://www.estadio.latinowebs.com/photo2_1.html

"Pessoa es ya un hombre de tertulia, un hombre que discute, que ironiza"
http://www.losandes.com.pe/Cultural/20100808/39414.html

"Mercedes, con un carácter y personalidad tan independiente, con la cultura como posesión más valiosa, tenía las ideas tan claras y una ética tan inquebrantable como su mala leche ante lo absurdo y banal. Mujer de tertulia y debate"
http://oriolserra.blogspot.com/2010_04_01_archive.html