I finally joined LinkedIn about a year ago, after lots of invites. I got one from someone I sort of communicated with anyway, and I wanted to see what it had to offer. What I've learned is that it's like most other internet social sites out there: it clearly states not to just invite everybody, and it clearly states not to just accept everyone's invitation...and then people just do what they want anyway. The reasons I've stayed with it:
1. As a writer, you'll benefit just by having your works, websites and blogs mentioned on your profile page. Then, whenever you comment on something that you really do want to comment on, your icon shows up, and if someone found your comment interesting or helpful, they can click on your icon, see your works and sites, and now you have another customer, or blog viewer. You'll immediately see the difference between amateurs and pros. Stick to the latter. To that end,
2. About 95% (and that's being nice) of the stuff that comes your way is unworthy of your time--but 5% isn't, and that's the nugget you swill for. Every great now and then, someone will say something helpful about blog traffic, or an agent, or you'll make a business contact, etc. When you find something interesting, you learn from it, you comment on it, and you're off. I met an editor of an anthology this way, and was able to write and sell a piece to her. That's what LinkedIn is really for--and you have to very quickly sift through the chaff to see something sparkle. I get the weekly feeds from my groups--all 40 or 50 of them!!!--but it takes me no longer than an hour a week to go through them all, make the comments that I want, meet the people I want, etc.
The important thing is to not let yourself get carried away. The most important thing about promoting yourself is to have something worth promoting. That means, write. Finish what you're writing. Send it out. The best marketing tool you've got is your work. Make sure you've got enough of it. Don't blog more than you write.