Twitter is very easy to automate, and partly due to this is appears to be a magnet for spam (or spambots). The vast majority of accounts are just this.
We experimented a bit for one town site, and rapidly built up a few thousand followers for a fairly small town. However, they were mostly bots or irrelevant to the town and as such useless. We scrapped it and started again, but this time we did not "follow back" and actively blocked any followers that did not appear real. Now it is taking ten or twenty times as long to build followers (we are only at 150, whereas before we had nearly 3,000). We tweet and retweet items that we think will be genuinely interesting to residents of the town, and link to our site a lot less. Both ways though it has produced little in the way of traffic, but at the moment there is very little on our site that is tweet worthy!
There are a couple of good feeds covering Cardiff for the general public (I know this is only one of your areas, but it happens to be local to me).
@GdnCardiff (the Guardian's Cardiff site) I think does it well, has far more followers than following, but still only a small number at 1,100 odd. Another is
@Buzz_Magazine (an old and well established local events magazine), who similarly doesn't use the follow back trap.
All IMHO of course, and I'd be very interested in the views of others using Twitter. I assume you are looking at Facebook also?