I know it's very common to use popup windows for such links. But does it have any advantage over a simple target="_blank" attribute?
I can see this being an instance where the user wants to share but doesn't intend to actually visit Facebook etc. Popups are popular but because of popup blockers, they're often not really popups (not whole new windows). Instead, they're little divs that appear and sit over the page near where the user clicked and load the social media stuff via Ajax (using the social media site's API to deal with cross-domain).
If this isn't a possibility for you for whatever reason and you don't want to use one of those plugins (shareThis, addThis) then between the two I'd use target=blank. Why? Users can configure if that actually opens a new copy of the browser or just a tab, and like Felgall can override either so right-click works (most users, being trained on the social media plugins to expect a small popup, will not think to try a right click though). Many browsers default to a new tab (though not IE it seems). Users are nowadays pretty familiar with switching between tabs. The main issue with this is, will the user always know a new [tab, window] opened? Also navigating to and from either a new window or a new tab is a break from whatever the user was doing on the original page, which is bad.
But I'm assuming you are choosing between two bad options
In general, yes, though this is a special case. People have had the experience of social media "Like" and "Share" and "Tweet this" buttons already shoved down their throats. Here, they already have certain expectations, so the OP should try to fulfill them as much as possible.