I’m in the invite-only social network Wallop, a Microsoft baby. I don’t think they’re being too picky about who requests to join, and I got in the old-fashioned way: begging.
Its interface is much funkier than those of any of the other dozens of networks I’ve tried out lately, but I’m not sure just what would keep me around there long (I guess the same could be said about 98% of them). If you want an invite, email me, either at marketersstudio (at) gmail (dot) com or dberkowitz (at) 360i (dot) com.
 
Below is a list of many of the broader social networks, as opposed to those centered on affinities, interests, demographics, or other niches. If you want regular updates on social networks, check out Mashable.com.