My understanding is that Steemit is currently in a bootstrapping/testing phase which is scheduled to end on July 4th. During this phase all author and curation awards are being funded out of a fixed pot of money (which I believe is $2 million), all of which gets paid out only once on or shortly after July 4th. So, as more people post stuff and more curation occurs between now and then, that fixed pot of money gets divided up among more people/posts. So, the value of each post therefore declines as each post's share of the pot is diluted.

However, after July 4, this all changes (or so I understand). At that point awards will no longer be funded out of the fixed pot but rather out of "Steem" that gets created ("mined" each day. Steel is the digital currency that gets "mined" by blockchain miners who secure the network. And, after July 4th, payouts to authors and curators will happen much more frequently (daily, as I understand it).

Once payouts occur on a daily basis, dilution will effectively cease or at least become much less of an issue. Again, that happens on July 4th.