No, no it isn't. I do not believe the interweb, but I do know NFL rules because of my past. And yes, I have seen it multiple times, usually when a LB gets caught behind a RB running a route up the sideline and he is beat. That said, nowhere is it posted that faceguarding is illegal.
SECTION 5 PASS INTERFERENCEARTICLE 1. DEFINITION. It is pass interference by either team when any act by a player more than one yard beyond the line of scrimmage significantly hinders an eligible player’s opportunity to catch the ball. Pass interference can only occur when a forward pass is thrown from behind the line of scrimmage, regardless of whether the pass is legal or illegal, or whether it crosses the line. When the ball is in the air, eligible offensive and defensive receivers have the same right to the path of the ball and are subject tothe same restrictions. Acts that do not occur more than one yard beyond the line of scrimmage are not pass interference but could be offensive or defensive holding (see 12-1-3 and 12-1-6).Defensive pass interference rules apply from the time the ball is thrown until the ball is touched. See Article 2 for prohibited actswhile the ball is in the air.Offensive pass interference rules apply from the time the ball is snapped until the ball is touched. See Article 2 for prohibited actswhile the ball is in the air and Article 4 for prohibited acts prior to the pass.ARTICLE 2. PROHIBITED ACTS BY BOTH TEAMS WHILE THE BALL IS IN THE AIR. Acts that are pass interference include,but are not limited to

a) Contact by a player who is not playing the ball that restricts the opponent’s opportunity to make the catch;(b) Playing through the back of an opponent in an attempt to make a play on the ball;(c) Grabbing an opponent’s arm(s) in such a manner that restricts his opportunity to catch a pass;(d) Extending an arm across the body of an opponent, thus restricting his ability to catch a pass, and regardless of whether theplayer committing such act is playing the ball;(e) Cutting off the path of an opponent by making contact with him, without playing the ball;(f) Hooking an opponent in an attempt to get to the ball in such a manner that it causes the opponent’s body to turn prior to theball arriving; or(g) Initiating contact with an opponent by shoving or pushing off, thus creating separation.ARTICLE 3. PERMISSIBLE ACTS BY BOTH TEAMS WHILE THE BALL IS IN THE AIR. Acts that are permissible by a playerinclude, but are not limited to

a) Incidental contact by an opponent’s hands, arms, or body when both players are competing for the ball, or neither player islooking for the ball. If there is any question whether contact is incidental, the ruling shall be no interference;(b) Inadvertent tangling of feet when both players are playing the ball or neither player is playing the ball;(c) Contact that would normally be considered pass interference, but the pass is clearly uncatchable by the involved players,except as specified in 8-3-2 and 8-5-4 pertaining to blocking downfield by the offense;(d) Laying a hand on an opponent that does not restrict him in an attempt to make a play on the ball;(e) Contact by a player who has gained position on an opponent in an attempt to catch the ball; or(f) When a team presents a punt formation (3-17-7) and before the ball is kicked, acts that normally constitute pass interferenceagainst the end man on the line of scrimmage or against an eligible receiver behind the line of scrimmage, who is aligned orin motion more than one yard outside the end man on the line. Defensive holding and offensive pass interference rules stillapply.ARTICLE 4. OTHER PROHIBITED ACTS BY THE OFFENSE. Acts that are pass interference include

a) Blocking more than one yard beyond the line of scrimmage by an offensive player prior to a pass being thrown. See 8-3-1-Note for exception for an ineligible offensive player; or(b) Blocking a defender beyond the line while the pass is in the air if the block occurs in the vicinity of the player to whom thepass is thrown. See 8-3-1-Note for exception for ineligible players