Are you more successful if you publicly commit to pursuing a difficult goal? If you have an ambitious goal, does it help to publicly commit to that goal? For example, if someone is training for a long distance triathlon but doesn't know whether he/she will be able to pull it off or not. This person could tell his/her friends about my goal or post it on Facebook. What consequences does it have for people, if they do that? On the one hand, it might push them more, because they don't want to be embarrassed in front of others if they fail. On the other hand, this is a pretty negative focus and might be distractive.
Ok, I found something. Gollwitzer and colleagues (2009) have published an interesting paper on this question. 

First of all, they say that, yes, publically stated intentions should lead to commitment, because the person wants to act in way that is consistent with the communicated intentions (e.g., Cialdini, Wosinka, Barrett, Butner, &amp; Gornik-Durose, 1999). Second, public intentions should increase accountability (Lerner &amp; Tetlock, 1999), which should also lead to an commitment effect.

However, they show that sometimes this can go wrong. Such public commitments may often be made in domains that are highly identity relevant and may thus be a means to reach some overarching goal. For example, committing to a triathlon may serve the goal of being a successful athlete. Now, symbolically, publicly committing to triathlon training may already help reaching this identity goal (because other people take notice of this effort). And thus ironically, it may be less necessary to actually reach the goal and diminish goal striving.

In line with these ideas they found that


  Identity-related behavioral intentions that had been noticed by other people were translated into action less intensively than those that
  had been ignored (Studies 1–3). This effect was evident in
  the field (persistent striving over 1 week’s time; Study 1)
  and in the laboratory (jumping on opportunities to act;
  Studies 2 and 3), and it held among participants with
  strong but not weak commitment to the identity goal (Study
  3). Study 4 showed, in addition, that when other people
  take notice of an individual’s identity-related behavioral
  intention, this gives the individual a premature sense of
  possessing the aspired-to identity


I should boast with the finished triathlon on Facebook only afterwards, I guess. 

References

Cialdini, R.B., Wosinka, W., Barrett, D.W., Butner, J., &amp; GornikDurose, M. (1999). Compliance with a request in two cultures:
The differential influence of social proof and commitment/consistency on collectivists and individualists. Personality and Social Psychology Bulletin, 25, 1242–1253.

Gollwitzer, P. M., Sheeran, P., Michalski, V., &amp; Seifert, A. E. (2009). When intentions go public: Does social reality widen the intention-behavior gap? Psychological Science, 20, 612–618. doi:10.1111/j.1467-9280.2009.02336.x

Lerner, J.S., &amp; Tetlock, P.E. (1999). Accounting for the effects of accountability. Psychological Bulletin, 125, 255–275.