Livejournal is my preferred platform still. I normally read all my friends-list, at least skimming it fairly continuously. I also use my friends list as an RSS feed, so catching up with a number of other blogs that way. I will generally add people to my f-list if 1) I have met them in person or 2) they write interesting stuff (or both). I sometimes post locked entries on a restricted filter for the first category, and more rarely on an all-friends filter. As they say, every day is defriending amnesty day for me, and if you take me off your list I will not make an issue of it if we ever meet in person (or in comments on someone else's journal).
All my public LJ entries get mirrored to Facebook as Notes. Not everyone appreciates this and I will not take offence at people defriending me because they find I am posting too much that they don't want to read (or for any other reason). Occasionally my posts generate more discussion on Facebook than they do on Livejournal (most recent example: the Bastille Day incident, which is the immediate reason for this post); but I'm not as enthusiastic about using it as a discussion space because it's only a semi-public forum, and archiving is much less straightforward. Also I have to admit that I check in with Facebook rather irregularly - once or twice a day, but I don't go the whole way back along my friends-feed, so I will miss things. I am a bit more restrictive in my friending policy on Facebook, and will not add people with whom I have had no previous interactions.
My public LJ entries also get tweeted in summary on Twitter, and I also tweet to update my Facebook status (using the selective Twitter application and the #fb tag). I do not check Twitter every day, and responses to me there may go unnoticed for several days. I will only follow people on Twitter if I know who they are, but of course anyone can follow me (I will block obvious spam accounts).
I am on Dreamwidth but using it only as a backup for my Livejournal, given that my archive is apparently too big for LJarchive to cope with. I check my Dreamwidth reading list once a week at most.
(Also using LinkedIn and a couple of others, but the above are the main ones.)