It sounds like the life of an expat in SD has not changed all that much since I moved back from there in 1999. As others have told you, SD is a large city with the expats a tiny portion and spread out, whereas in place like Sosua they are concentrated, are a large percentage and tend to all know one another. There is no single place or group of places to find expats in SD that I know of, whereas everyone knows well where to go in places like Sosua or Puerto Plata.
At the risk of over-generalizing, most people go to live in SD for different reasons than many on the North Coast -- i.e., they're there because of work or family or both, and this eats up much of their time, energy and social focus. That certainly was the case for me when I lived there.
I found several types of expats in SD when I lived there: (1) the diplomats and local top company execs who thought that they were too good for us regular folk, even if they would have been just like us back home; (2) the transients, who were many, coming to SD making a good show of settling in, only to disappear without warning a few weeks or months later (can't count how many of these I saw come and go over 4 yrs); (3) the "questionables" (met many of these), the ones people in this thread have warned about, who are running from something or are just shady and with whom you always feel (or should feel) on your guard; (4) the "lost souls" -- regular people who slowly find each other, often by accident, over time.
Careful about seeking out expats too much -- it becomes a crutch, and you don't learn Dominican ways and thinking, speaking, relating. In my experience, the people who hung out only with expats made little effort to understand the host culture and viewed it through a warped lens. That doesn't mean you should shun them, but understand that you need to find your own "fit" in the Dominican scene through direction interaction with Dominicans as much as possible.
As for places... well, a suggestion or two. If you like cultural events, check with the
Dominico-American to see what they have scheduled or know of, and they should have a mailing list. If you like books, check out the Helen Kellogg Library, located at Av. Independencia 253, behind the Ephiphany Church, Tel. 689-8949. Someone else mentioned Carol Morgan -- I would follow that tip up. Also, check to see what other tips DR1 has at dr1.com/living
Hope this was helpful.
Best regards,
Keith R