As someone who's had a couple of posted chapters of my story 'liked' rather than responded to, I can say from experience that it's... kind of a nothingness, really. It does allow you to gauge whether or not anyone enjoyed it enough to stay on the page for more than half a second after finishing the chapter, but as far as incentives to keep writing go, it isn't as effective as even the most cursory response.
However, that's not the question under debate here. The issue is whether or not it's a good way to gauge reader's interest, and, given the state of reviewing on these forums, I have to say that I think any form of response, even one as indirect as a 'like', would be of interest. The danger is, of course, that reviews lapse in favour of 'liking' - but honestly, since virtually no one is reviewing anyway (and here I'm as guilty as everyone else, and more so than some), I'm not sure that that's going to be a problem.
You raise the point that someone could conceivably go through all the threads in the forum and 'like' the lot without reading them; this is technically true, but realistically I'm not sure that anyone would bother doing that. That kind of scheme would be relatively easy to discover, as well, for anyone who regularly checks on more than one story.