The graph below illustrates the trends in how teams favor each position over the past 41 years. Most positions are fairly stable. Click to expand.
I take this as a pretty clear indication of the relative values and scarcities of each type of player at the elite level. As passing becomes more and more prominent in the sport, and as teams realize that the success of the running game depends less on the singular talent of the ball carrier than on other factors, fewer RBs are selected.
So what positions are making up the difference? Defense backs primarily, having gone from 12% of first rounders in the '70-'74 period to 18% in the '05-'10 period. This squares with the ascent of the passing game.
One curiosity is the very high rate of selection of defensive linemen compared to offensive linemen. There are often four, and sometimes three, defensive linemen on the field at a time, and always five offensive linemen on the field. But defensive linemen are consistently selected more often than their offensive counterparts.
Notes: I included fullbacks as RBs in this analysis, but there were an insignificant number of them chosen in the first round: 1 in '80-'84, 1 in '85-'89, and 2 in '90-'94.
Data are from the Pro-Football-Reference.com Play Index Draft Finder.