bad river fold?

[lead24]

SB: $31.35
BB: $172.75
UTG: $48.65
CO: $49.25
Hero (BTN): $79

Pre-Flop: J[img]/images/graemlins/heart.gif[/img] A[img]/images/graemlins/diamond.gif[/img] dealt to Hero (BTN)
2 folds, <font color="red">Hero raises to $1.75</font>, SB calls $1.50, BB folds

Flop: ($4) 7[img]/images/graemlins/heart.gif[/img] 8[img]/images/graemlins/heart.gif[/img] J[img]/images/graemlins/club.gif[/img] (2 Players)
SB checks, <font color="red">Hero bets $2.50</font>, SB calls $2.50

Turn: ($9) K[img]/images/graemlins/club.gif[/img] (2 Players)
SB checks, <font color="red">Hero bets $6</font>, SB calls $6

River: ($21) 4[img]/images/graemlins/spade.gif[/img] (2 Players)
<font color="red">SB bets $21</font>, Hero folds

Results: $21 Pot ($1.05 Rake)
SB mucked and WON $19.95 (+$9.70 NET)

very early in the session. I wasn't running PAHUD so no stats. I know my bet sizing was small but i'm interested in his river play. Is that a bluff enough to justify a call?