I pop in at least 5 times a week (M-F), and typically a couple of times a day. If the threads seem covered, or it's something I'm not interested in (or don't know about), I don't post. I do get frustrated with slow sites, ads, and hard to navigate sites, and sometimes I just don't want to "deal". Cranberry and I go to great lengths to keep our own site up and running properly for that very reason.
Lots of peeps are indeed getting out of the hobby or are making the switch to FOWLR setups from reefs, which is another reason some reefing sites are slow as many lack good FO or pred forums. I also think many folks are tired of hearing regurgitated old info as opposed to first-hand experiences, which means that folks are becoming more picky about where they spend what little time they allot for such activities.
Finally, folks are planning summer vacations, graduations, etc. this time of year, and we all know "life" generally comes first.