I don't think that there are fewer people participating (although I have not measured this since I am not sure what the right measure should be). It could be that we are getting more questions per day and that spreads people more thinly. Or perhaps it is all a perception of what one particular individual observes but does not reflect on the entire site.
Usage wise I've always seen a continuous and steady growth. To give you a sense in May 2011 we had 53K visits, 27K unique visitors and 132K pageviews. By May 2012 we had 118K visits, 64K unique visits and 242K pageviews.
Every metric that I have ever measured show a a steady and continuous growth in all measured parameters. After the first year we already had doom-and-gloom posts that stated that the Biostar is fading and things are going south - amusingly since then we have grown at least an order of magnitude.
I always feel sorry when someone gets upset with this or other parts of the website. We are trying hard to make the experience as positive as possible. Even then some may find that the site does not operate the way they wish it did - I grew the accept that as the way the human spirit works.
Sometimes the people that are unhappy and restless go on and create a better experience. That's why Biostar exists in the first place and thats why systems better than it will be created in the future.