Like new FB pages
But I also think that liking, then commenting on, new pages with low numbers can be beneficial. This is because these page owners really appreciate the fact that you've actually contributed to their page. They're more likely to reciprocate by liking yours back, or commenting on it, or liking your posts. (At least that's what my own recent experience tells me.)
I also suspect that they're much more likely to continue to engage with your page, particularly if you do the same with theirs. (It's a bit like a couple of kids who make friends on the first day of school. It's a strong bonding experience.)
Comment on young blogs
This also holds for blog commenting. Of course you should comment on prominent, high traffic blogs, since they'll tend to bestow more clicks than the younger ones. Still, how many of those popular bloggers will have the time or the inclination to have a good look at your blog, let alone comment on it?
Certainly some of them will do this. But it won't be most or all of them. Unless you're consistently writing absolutely brilliant comments on their blogs, the big names are unlikely to check yours out.
New bloggers, however, being chuffed that you have responded, will almost always do this. They'll often come back to comment again and again, too.
Potential long-term benefits
Remember that even the big names had to start somewhere, too. So a certain percentage of the newbies that you connect with via mutual commenting will eventually become established bloggers in their own right. You might end up benefiting traffic and link wise at some stage way down the track.
For these reasons it's a good idea to engage with a mix of new blogs and Facebook pages as well as old and established ones. You'll get the best of both worlds then.