Congregations of Jehovah's Witnesses run small: right around a hundred. They expect that hundred to show up frequently and part of their pattern of worship includes one night (or day) a week meeting as an even smaller group for deeper study of WTBTS publications. They've tended to build their own facilities whenever possible and, in keeping with a DIY culture, they have organized volunteer crews to build those facilities. It has not ever been uncommon for a Kingdom Hall to be retired to be replaced with a new one, but this typically did _not_ happen because a particular congregation got larger. Congregations which got big enough were instead split (not generally into two, but two might be split into three, for example). New halls have generally been built for newly settled suburban areas, to replace repurposed halls that didn't work well JWs, aging facilities and, occasionally, because the local JW population is booming and they've already got 3 congregations in each existing facility so they build a new one and let everyone spread out a bit (wedging more than 3 2 hour services in on Sundays is _hard_ -- on parking ballet if nothing else).
When I stumbled across multiple closures of purpose-built Kingdom Halls, I was a little surprised: they weren't old, they weren't going to be replaced so the most likely explanation is that, in the same way that congregations might beget new congregations, congregations can be combined when they shrink below a certain size, and money can be saved on keeping a facility active by combining congregations into a smaller number of facilities. This happened when I was growing up in parts of Seattle that depopulated in the move to the suburbs and also in parts of Seattle that became, overall, too wealthy to support the existence of JWs (JWs are very poor. Pew says so. I could tell you why, but it is boring. Let's just say they like it that way and call it good.).
Here's the first coverage I've seen by a pro-journalist that explains what happened to a particular Kingdom Hall (I'm still looking and I'll add more when I find it):http://www.patriotledger.com/lifestyle/faith/x686278928/Marshfield-religious-landmark-up-for-sale
A quick look at Furnace St, Marshfield, MA suggests that the hall was there because it was close to a cloverleaf on route 3 (thus could draw from a substantial area), but not so close as to be expensive real estate. The article indicates the congregation (singular) merged with one in Rockland, 12 miles on back roads, 15 on route 3, no matter how you slice it, a half hour distance between the two halls (which explains why they both existed in the first place). Marshfield is ancient, storied and not poor; the combined size of the congregation according to the article is 110.
I would not generalize anything whatsoever from this. JWs are thin on the ground in the Northeast. Getting thinner is not unexpected.
There are, however, a scattering of Kingdom Halls up for sale on LoopNet:http://www.loopnet.com/Listing/17161797/99-Kingdom-Hall-Dr-Clyde-NC/http://www.loopnet.com/xNet/MainSite/Listing/Profile/Profile.aspx?LID=16698124
That one is at 2950 Arizona Ave, Washington DC
Here's a weird one:http://www.loopnet.com/Listing/16784429/2309-N-Broad-St-Philadelphia-PA/
Photo suggests it was originally the "Philadelphia Electric Company" and had spaces upstairs and down.http://www.loopnet.com/Listing/16958273/1960-Memorial-Parkway-Fort-Thomas-KY/
That one is a very few miles from Cincinnati's CBD.http://www.realcommercial.com.au/property-other-nsw-woy+woy-5794927
That's in Woy Woy, New South Wales, which is north of Sydney.
Here's a sample of what ex-JWs think of some of this activity:http://www.jwsupportforum.com/index.php?topic=4661.0
I tend to think that if the hall is located in an area where the land is valuable, there probably aren't that many JWs living nearby anyway, and it's easy enough to tell the few dozen who are left to combine with a nearby congregation which is also small. Does this mean JWs are ending? Well, in the Northeast, sure, in richer areas, sure, in urban areas? As they gentrify, sure. Heck, they may be on their way out in the US as a whole. Do I believe that this is fundraising to defend against lawsuits the way some of the posters above think? No. But you never know.
If you would like to play this game with churches in general, you might try starting here:http://www.loopnet.com/Churches-For-Sale/